I think a big part of it is liberals believe they are intelligent, free thinkers who can sift the truth from the lies. Trouble is, they are not given the tools to do so, and liberalism is ingrained in people from the moment we first learn to talk, so it comes as such a natural thing to them that questioning the liberal outlook would be like questioning the existence of the sun to them. It's just a thing that is there, a permanent fixture of life.
So they believe they are determining the "facts" correctly, but they don't even consider that the framework they use to examine the facts is flawed and doesn't always give correct results. It just "is." and by extension, everyone uses the same framework, because it is the only framework that exists. So when they see someone who arrives at radically different conclusions to themselves on an issue, they have two options in their mind:
1: I was using the framework wrong.
2: They were using the framework wrong.
So when they look at people like QAnon, this is an easy answer, as they also use the liberal framework, but clearly have "gotten things wrong." so to your standard lib, they can point that out no problem, as QAnon is easy to debunk. And in the event of a situation where they are wrong, but within the framework of liberalism, they can have no problem admitting that (some do, but plenty of "skeptics" and that tend to be willing to change their minds on things within this framework.)
But when they encounter someone outside the framework, like a communist, they just don't understand. We don't fit into either of their two boxes, because we aren't using the same liberal framework to analyse the world. So they awkwardly try to fit us into the "Using the framework wrong." box, but we obviously don't fit into it neatly enough. Worse still, things that liberalism says are "impossible to predict" or "impossible to prevent." are both predictable and preventable within a socialist framework. Things like recessions happen like clockwork under capitalism, but the liberal response to this is to shrug and say "no one knows how they happen." So when we predict a recession, or a socialist country doesn't suffer from one, it causes the liberal a lot of discomfort. We're supposed to be using the framework wrong, yet here we are, understanding an event better than they do.
So their only option is to insist we are "lying" or "fake news" somehow. When liberalism brushes up against reality, they have to choose liberalism or reality. Most will just plug their ears and force themselves to watch Marvel movies, Clockwork Orange style until they forget all about it, but some realise the system they've taken as a gospel truth their whole lives isn't actually a good framework for understanding the world, and they might like to look for better ones. (Unfortunately, a lot of them fall for fascism at this point, because it offers the same easy answers as liberalism, but pretends to not be liberalism. So it is familiar and comforting to them.)
Jeez, take a shot every time I said "framework" goddamn.