Historically, the "flat earth" theory is actually incredibly modern, it doesn't appear in any texts until the late 1800s and even then it's only used facetiously to mock religious people in a "they're so stupid they actually believe this" way.

It was literally meme magicked into existence at first by fucking Looney Tunes and then subsequently picked up by grifters who probably likely just wanted to see how far they could take things and now we're at a point it's functionally a cult.

It was literally a joke. It was ALWAYS a joke.