Because other ideologies, like liberalism, don't pretend to be a completely universal, internally consistent, axiomatic moral framework. Liberal philosophers, from Mill to Jefferson, understood and accepted that the balancing of rights equities was not something that could be achieved through *a* *priori* rationalization. But that, it would need to be reevaluated over time, and evolve.
AnCaps mostly cast this insight aside and say: that's wrong. All you need is self-ownership and the non-aggression principle.
Then, when someone like me intuits pathological cases that can emerge from those axioms, dismissing those cases as being incredibly unrealistic because of the human propensity for "cooperation" is just inadequate.
If humans already had this in-built propensity in our nature, then why are we even where we are right now? Humans seem to have just as much of a propensity for seeking out alphas, developing in-group/out-group biases, and falling victim to demagoguery, as they have a propensity to cooperate in groups.
When people start saying things like "no, that's just due to the distortative influence of the state and/or fiat currency" -- without providing a rigorous challenge to the counterfactual, I start to just assume I'm dealing with dogma, goal-seeking and confirmation bias.