Why do people have beliefs that are clearly ridiculous?

I ask because I just don't understand this; I'm genuinely confused. For example, the belief that many now apparently have that the Earth is flat! Seriously? Are these people just trolling us? Is this all some big joke that I'm not aware of?
My sister often watches this YouTube channel, and I have to say it is full of batshit crazy nonsense of the highest calibre!

https://www.youtube.com/@minutesofhorror/videos