The primary reason for not being an Objectivist is because the philosophy can't deliver on most of its promises. Objectivism claims to (1) show a path to the establishment of laissez-faire capitalism; (2) provide an articulable philosophy-based ethics to provide guidance for achieving one's values; (3) show you how to think in ways that our "reality-orientated." Let's examine these one by one.
(1) If you are an Objectivist who believes that Rand's philosophy can help bring the political society of your dreams (i.e., laissez-faire capitalism), you are mistaken. Objectivism can do no such thing. Rand believed that the politics of a society could be changed by arguing about abstruse points of philosophy. In order to believe that, you have to accept Rand's blank slate view of human nature and her theory that emotions are automated value-premises originally "programmed" into the sub-conscious mind, whether by chance or conscious focus. Both these theories are false. The sciences of human nature have refuted the blank slate view many times over; and emotions are not "automated" value judgments --- they constitute one of the brain's several cognitive systems, providing information necessary for reasoning and making moral decisions.
Objectivism has had more than seventy years to change the culture and politics of American society. Even though as many as eight percent of adults in America have read Atlas Shrugged, what has Rand's philosophy achieved over that period? Anything at all? Has freedom increased over the last seventy years? Has the government shrunk and become less corrupt? Has the radical left become less influential over society? Is our culture less or more open to new ideas? Is freedom of speech as secure as ever?
The fact is, the course of society is not determined by the wranglings of philosophers, as Rand and Peikoff assumed. If the West is in crisis, it is not because of Immanual Kant. Hardly anyone understands Kant, and there is absolutely no evidence in either psychology or historical sociology that abstract philosophical constructions effect people (particularly elites) in such a way that they can be said to determine the course of history.
If philosophy doesn't determine the course of history, what does? That's a very complicated question, well beyond the scope of a single blog post. But we can glean a hint of the process by examining how the "woke" left has become so influential in our culture in recent years. The number of people who identify with his peculiar brand of leftism is somewhere between five and eight percent of the population, and yet despite these modest numbers, these radical progressives exercise a disproportionate impact on the culture and even society. How has the radical left accomplished this?
One explanation is that they have conducted a "march through the institutions" --- that is, they've taken over many key institutions in the culture, especially the universities and mainstream media. This is of course an exaggeration. The radical left hasn't in fact taken over anything really. But they do exercise a surprising amount of influence. How do they do this? To give a very brief and very rough answer: they know how to make such nuisances of themselves that the people who actually run these institutions (neo-liberal types mainly) give in to them so they can be left in peace. In societies where violence is largely absent (particularly among elites), power tends to pass from the strong to the passionately manipulative. The radical left has achieved power well beyond their numbers because they just want it more and in a thoroughly civilized society (i.e., a society where force has been removed as a factor in the competition for preeminence) power is often attained by those who demonstrate the most passion (i.e., through manipulative large scale temper tantrums). In societies where powerful elites can inflict violence on all challengers we don't find anything like a "woke" left. There exists no such radical left in authoritarian societies like China or Russia. The Chinese elite laughs at American "wokeism" --- as do the elites in authoritarian societies all over the globe. Removing violence from society is a great achievement, but every silver lining has a cloud.
Now could Objectivism achieve comparable success to the woke left if Rand's follower's followed the left's example? If they started infiltrating institutions of culture; started making a nuisance of themselves any time they didn't get their way; engaged in vicious attacks against the elites of these institutions; began accruing enough influence that they could use a form of social coercion (i.e., destroying people's reputation) to get what they want: could admirers of Rand behaving in this fashion create an Objectivist form of cultural hegemony? Of course not --- and for very good reason: few if any Objectivists are hardwired in that way. In order to conduct a "march through the institutions," Objectivists would first have to infiltrate these institutions; which would mean pretending to be "woke" leftists until enough of them had advanced to key positions of the administration from which they could impose their will on the entire institution. Now realistically this is not something Objectivists could ever pull off. Fans of Ayn Rand who admire the integrity of Howard Roark aren't going to spend years of their lives pretending to be woke leftists so they can take over Universities and Hollywood. That would involve too high a degree of self-mutilation to even consider, let alone carry out.
(2) The Objectivist Ethics cannot provide a guide to life because morality doesn't work that way. We know this because of psychological studies that demonstrate the extent to which "intuition" determines moral decisions and judgments. As Jonathan Haidt has put it, in morality, the intuitions come first, then rationalizations (i.e., moral philosophy) is concocted later to put a rational veneer over what originally has, and must have, a non-rational source. Morality must arise largely from intuitive sources because it turns out that many moral decisions are too complex to leave to the cerebrations of the conscious mind. They involve harmonizing an ever-shifting, ever-adapting hierarchy of values to the convoluted and unpredictable situations and dilemmas faced in everyday life. Deriving moral decisions from such hierarchies involves more complexity than the conscious mind can handle. (Rand should have understood this as a consequence of "unit-economy," but she never the made the necessary connections.) Morality should be seen, not as a series of rules or values that are applied to simplified constructions of reality (which is how ethical philosophy views morality), but rather as a decision matrix of labyrinthian complexity which the adaptive unconscious uses to provide instantaneous moral intuitions. (There is quite a bit of empirical evidence from experimental psychology supporting something along these lines.) Out of this decision matrix the human mind attempts to figure how best to maximize the individual's hierarchy of values given the restraints placed upon individual by the both the natural and social environments.
The Objectivist ethics is far too simple a construction to replace this insanely complex intuitive decision matrix used by the adaptive unconscious to help guide the individual through life. Since Rand's ethics is not rich enough in sophistication or scope to provide such guidance, Objectivists who try to follow Objectivism morality have no choice but to draw on their intuition to make decisions whenver they face any great level of complexity. Then they use their conscious minds to harmonize the results of this intuition with Rand's ethical notions. Since there's a large amount of rationalization involved in this exercise, it turns out that trying to follow a consciously directed ethical philosophy like Objectivism is not an altogether honest or self-aware exercise.
(3) Can Objectivism help you understand reality? Is Rand a great paragon of "reason" who can teach you how to think? The answer to both the questions is an equivocal no. Just as our moral decisions and judgments cannot be guided exclusively (or even primarily) by an explicit ethical philosophy, so human cognition cannot be guided exclusively (or even primarily) by an explicit epistemological philosophy. The human brain has evolved several very sophisticated cognitive systems that have been fine tuned by centuries of experience to help human beings navigate the complex social and natural environments of everyday life. While these cognitive systems may be a bit of an evolutionary kludge, since they are actually adapted to the complexity of the real world, for that reason alone they will often produce more useful output than consciously directed thought. Because of the conscious mind's incapacity to grapple with complexity, emotive centered systems of cognition arising from the cognitive unconscious (i.e., "intuition") often must be used instead. Not necessarily in all circumstances, of course. But in quite a few. There is nothing in the Objectivist epistemology that will help you with "intuition." If anything, Rand looked down upon intuition as "emotional"; and Rand, as is well known, did not consider emotions a "valid" form of cognition.
Science, when done right, provides us with our most reliable knowledge. But science is a rather cumbersome and expensive instrument: it cannot be relied on to solve every riddle of existence. Objectivism claims to be scientific --- but there's also a strain of anti-science running through Rand's epistemology. Rand seems to have believed that the "Kantian corruption" had run so deep in Western culture that it had to be affecting science as well as culture. This troublesome notion becomes weaponized in the Objectivist insistence that philosophy enjoys a veto power over science. Now we could imagine scenarios where some body of thought similar to "philosophy" --- something like, for example, a rigorous, empirical-based "philosophy" of methodology --- might have a veto power over some types of science (or rather claims that "science" is applicable to a given field of study, such as economics or history). Some denizens of science believe that science is the only valid means of knowledge and therefore seek to apply to the methods of science to all disciplines, even where such methods clearly don't apply. In that case we have something that can be described as "scientism." However, when Rand's disciples claim that philosophy can dictate to science, they have something far more controversial in mind. They wish to dismiss any of the conclusions of science that do not square with their naive Aristotlean realism. Such dismissals, they contend, are justified on the basis of the Objectivist axioms, two of which happen to be tautologies! If you are going to make a claim that philosophy has a veto power over science, you have to come up with something better than that.
To sum up: It's best not to be an Objectivist because (1) Objectivism can't succeed in its politcal goals; (2) Objectivism cannot provide an adequate guide to achieving one's values; (3) Objectivism cannot help you think better.