The problem doesn't stem from their critical thinking skills, but rather, a combination of indoctrination and religious beliefs which cause a series of presuppositions to supersede the conclusions the rest of us have taken as presuppositions.

In some sense I admire their dedication to challenge established facts about reality, that should be done in general.

To clarify the statement about indoctrination and religion: many people are not aware that flat earthers are religious wackjobs. Their thought process is as follows:

"If the universe contains trillions of celestial bodies and they're all spherical, then we are inconsequential. It must be the case that we are special, and earth is not a sphere, but is flat."

Some flat earthers will claim space doesn't exist, even though their bible tells them God created other planets.