It's hard to get into peoples' heads, but in my experience, I've known people who were into conspiracy X, but did not want to admit that they believe in conspiracy X at first, so they told their friends that they were just joking, being edgy or learning more about it, etc.

After a while, they just grow tired of keeping up appearances, or maybe a famous person talks about conspiracy X, so they are no longer embarrassed by their belief, etc.

It's a common misconception that other people think alike oneself, and that everybody think rationally. As always, it depends.

> but unfortunately it appears to quickly devolve into actual, very firm beliefs for many people.

Is it possible to quantify "many people"? Like, what percentage of people that are ostensibly flat earthers are actually genuine flat earthers vs. trolls? I would guess trolls make up the majority of internet flat earthers.

Trying to ascribe the intent of an action rather than focusing on the action itself is part of the problem in this circumstance. It's impossible to know how many are sincere, or how many are trolls pretending to be sincere, or how many are sincere but are pretending to be trolls so as to plausibly deny their sincere beliefs, etc. At the end of the day, the intent doesn't matter. If one is in a public forum (especially an anonymous/pseudonymous public forum) then the only thing to judge is one's actions, not their intentions, and if their actions are deliberately stupid, then we must assume they are simply stupid, and not merely pretending to be stupid. This approach obviously has problems, but there's no other tractable solution. The internet (especially Poe's Law) has killed satire.

Flat earther trolls that I saw didn't look sincere or stupid, they looked ironic.

"Irony", or what passes for it these days, is too often a deliberate tactic to equivocate a sincere belief while retaining plausible deniability. Unless they deliberately call out their comments with something like an explicit sarcasm tag, we are forced to assume they are sincere.

In my culture irony equivocates amusement, not sincere belief.

This may simply be a function of your inability to believe that they could be sincere.

There are without question plenty of "real" flat earthers our there. People willing to spend actual money on experiments doomed to fail. People cutting ties with their families because they don't agree with them.

When we read people espousing flat earth views on YouTube or Reddit or wherever, out natural assumption is to think that they must be trolling, because nobody could be that stupid, but in fact there really is a subculture that does.

You want to say irony must be imagined and can't be seen?