It's pretty much like believing the Watchtower. If you look only at one point of view, rely entirely on cherry picked bits of information and ignore reason, science and logic, then you can be persuaded to believe just about anything.
Conspiracy theories thrive because people want to imagine they are smarter than others and have special, inside knowledge. It takes a willful ignorance to maintain a belief in a flat earth.
Education is the antidote, spend an hour on Wikipedia looking at evidence of a round earth, then you will never fall for that particular stupidity.