Comment by kube-system

It is primarily this:

https://en.wikipedia.org/wiki/Anti-intellectualism

Mixed more generally with this:

https://en.wikipedia.org/wiki/Populism

Someone who is anti-intellectual doesn't believe that their theory of the world being flat (or whatever) is actually a rigorous idea. Being "right" isn't even part of the thought process. They have an issue with science and the intellectual process itself and see those who practice it to be an outsider to their worldview.

This is why so many people fail to convince flat-earthers that the world is round. Because they don't really give a shit about what shape the earth is, they just have contempt for the intellectualism that says it is.

I suspect many of these people are insecure about their own lack of knowledge, and so by rejecting a mainstream narrative of science they can feel in-control of their own sense of intellect.