Because without the network effect of adequate education, scientific understanding doesn't scale.

What does scale, unfortunately, is arrant nonsense.

I'm pretty sure people claim to be flat-earthers as a lark.

They aren't, what you see is a combination of trolls and propagandists; very few real human beings actually believe in such things.

You can't claim to have superior knowledge if you admit you're wrong.

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.

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This photo could be easily faked. I don't believe the Earth is flat, but I also don't think everybody on the planet should be 100% on one side of a discussion. Even if flat-earthers are kinda dumb, I think it's worse to force everyone into intellectual submission just because you're "right".