Maybe I'm just a dumb optimist, but I've always assumed that the flat earthers started out as an enteraining debate club intentionally trying to prove something impossible just for the challenge, which got overtaken by a tiny number of vocal idiots. I have heard they largely have moved on to Qanon, which tracks.
The initial modern flat earth movement was absolutely trolling, no doubt about it. But as the myth grew, enough grifters and actual idiots glomed onto the idea that it became what it originally mocked. Poe's law and all that.
Similar to the "I fell in love with an AI!" folks, it's largely undercover salesmen hawking their goods to the gullible.
