Probably because they look at her philosophy of Objectivism completely backwards. It is often incorrectly viewed as a top-down philosophy of politics affecting the individual. Instead, the best view is Objectivism is a philosophy of the individual, not a political movement. If individuals adhered to a more objectivist philosophy personally, then it would impact forms of government from the bottom up because there would be no demand for big brother pandering.
Understanding it from that direction completely changes some of the issues people have with Objectivism. Also, I should note that Objectivism isn't a religious philosophy that is supposed to be a 'perfect', instead it is more of a roadmap philosophy.
Plato taught me to look at it this way: is all individualism good?
The answer is, no. Now the job is to recognize what in individualism is anti-social and vicious, and what in individualism is good.
It struck me as being a species of philosophical egoism, of an extreme sort. Useful in small doses as a medicine to counteract Communist propaganda, but in larger doses a moral poison.
As a roadmap, it's a path to anarchy, albeit from a different route than that followed by traditional socialism and communism.
It's not just anti-Socialism and anti-Communism; it's anti-Society.
If followed to its logical conclusion, its result would be not a utopia but a wilderness. A wilderness of dragons.