[secretasiandan]

I would suggest that's because they don't (often) feel threatened by Plato or by the way people present Plato's views.

Also, the objectivists they're speaking to/at aren't always looking for dialog as much as "scoring points".

You can sometimes, with work and patience, introduce ideas that threaten someone's worldview. But I think people's efforts to do that are as rare as their efforts to objectively study ideas they aren't biased towards.