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I was about to make a post about this exact question. It fascinates me endlessly, to the point where I've probably watched more flat earth videos than most flat earthers.
The closest to an answer I've got is that I just think these people either believe they're smarter than the school system or society has ever given them credit for, or that they're aware they might be a little less intelligent on some level, so seek to grasp on to any conspiracy theory that could be plausibly true if you choose to follow one very narrow line of logic (such as "I can't see the curve").
The FE lot are always going on about 'observable reality', making out that if you can't see something with your naked eye, it mustn't exist. They also love to tell you whatever they've found is irrefutable, yet ignore or accuse anyone attempting to refute it as either an idiot or a NASA shill.
In short, I think it's a way to make themselves feel clever. They've worked out (aka been spoonfed an idea) the biggest conspiracy ever, and they're in on it and all the clever scientists are either stupid or evil. The only reason to do this would be to salve their wounded egos.
There also seems to be a religious element for some of them, because apparently the bible's description of the earth doesn't work with a round earth or heliocentric model. There's plenty of atheist FE nuts, though.