Dunning–Kruger effect. People will draw conclusions with whatever information they have on hand. The less they know, the less chance they will come across or understand something that will contradict their opinion. In the case of flat earthers, it's a lot easier to understand that the sun and moon look the same size and therefore are, as opposed to understanding the very complex concept that things that are farther away look smaller.
Basically, you need to be certain level of intelligence to even realize you're a moron. Anything below that and you think you're the smartest person in the world.