[cogman10]

I like to look into wacky conspiracies and where they come from.

Quite frankly, the most common reason people believe in a flat earth is because of biblical literalism. There are a few passages in the bible (which, if you ever watch a flat earth video, those almost always come out) which mention things like the earth having corners or god rolling it up like a scroll. Those verses are used as the grounding point for why the earth must be flat and all other evidence to the contrary is a lie.

This is also, consequentially, the origin of moon landing denialism. Mormons used to believe that the moon was literally a part of heaven. As a result, it'd be impossible for god to let someone fly a spaceship there. Pretty much exactly the same process happened "It couldn't have happened because our holy texts say the moon is the terrestrial kingdom... therefor it must be a hoax".