Why are they always larping as ancaps/libertarians?

Broadly, for the same reason that tankies larp as communists - because they're some combination of dishonest, deluded and blind.

Some are simply liars and schemers, and there's no real analysis needed there. But that's a minority - most appear to essentially mean well, but they just can't or don't grasp all of the necessities of the position they claim.

I think most of them start out as relatively sincere libertarians, which is to say, they're relatively sincere about opposing government overreach and advocating for individual liberty, at least speculatively.

But then they run up against the fact that other people don't share their views - that some don't even share their basic views and many more don't share all the specifics. And for whatever reason, they can't tolerate that - they're invested in their position and they can't abide the thought that another position not only exists, but could be valid.

That's exactly the point at which their opposition to institutionalized authority and purported advocacy for individual liberty should take over and lead them to simply accept the fact that other people have other views, and should be entirely free to do so, but they're just not psychologically equipped to do that. So they instead jump to the position that those other views need to be stamped out and/or that people should not be allowed to hold them.

And since those other people are rarely willing to relinquish their views, they then tend to jump to the position that they should have the authority to force them to do so.

And that's pretty much the end of libertarianism (or communism as the case might be) right there. It ends up with people who claim to be opposed to hierarchy and institutionalized authority not only proactively stipulating the institutionalization of authority, but treating it as rightfully theirs.

Sadly, I even see the same thing in anarchism - in fact, I see some variation on that dynamic far more often than I see people who actually hold to the ideal of eliminating hierarchy. It's simply that so many people think nothing of presuming the right to decree that other people should not be allowed to [X], apparently completely oblivious to the plain fact that the moment they do that, they're presuming that their opinion on what people should or should not be allowed to do supersedes the opinions of those who hold different views, and since those other people likely aren't going to voluntarily submit, they more often than not then jump to the position that they are rightfully forced to submit. And boom - right there, we already have hierarchy and authority.