The depressing fact that stupidity given a forum...Grown men thinking that the earth is flat...
Meanwhile David Mitchell has a great name for them...'Idiots'
This sort of nonsense seems to be driven by prioritising personal experience over scientific or governmental authority.
To the subjective observer, who is familiar with every-day objects like tables and marbles, the world is self-evidently flat - you walk along a flat surface, you don't fall off; you have a glass of water, the surface is flat, not curved, etc. Once you have made this your defining knowledge, then anything you are told about gravity, aeroplanes, space, etc sounds like a trick.
Ditto, the moon landings (it's too far away), 9-11 (I saw an explosion on TV), climate change (it was cold yesterday), etc.
But it does raise interesting (?) questions about how we know things. Why do we believe some of what we are told and not others?
It also may be the psychological rewards of being in a cult of any kind. It seems that humans are designed to prefer group conformity over individualism (in a tribe it is more important for survival to paint yourself blue, to show you are part of the tribe, than to say, 'what's all this nonsense about painting ourselves blue?' - this is one reason I only lasted 3 years at Barclays.) For some people, it feels better to be closely-aligned with a small tribe of nutters, than to be lost in the general mainstream.
(Although I suspect 50% of the flat earthers are just having a laugh.)