Do not underestimate a human's ability to delude themselves or, more importantly in this case (in my opinion) buy into being part of a social group. Flat earthers have a pretty active social community going online and some people will absolutely buy into the things they say just to be a part of it. And this isn't some 'I'll just pretend to so they'll talk to me' thing, they will convince themselves they're true believers. The belief in a flat earth is often downwind from the belief in other conspiracies, things that may be pretty benign or just outright true (Tuskegee experiment, MKultra etc), putting Flat Earth on a path from the benign to the deeply harmful, like Pizzagate or raging antisemitism.

In all this, community is extremely important. If a person loses this access to the community, their beliefs may turn somewhat more mainstream. We see this with cults too, though I'm not saying Flat Earthers are a cult by any means.

Anyway, there are Flat Earth books, podcasts, YouTube channels, conferences, you name it. There's a real dedication to showing everyone in the community just how much you bought into the thing. I think approaching this from the standpoint of 'this is a personal failure for the one believer' doesn't provide us with any answers about the 'why' of it all, nor will it necessarily help anyone with leaving such a worldview.

Anyway. My two cents. The best way to make sure someone leaves a cult or a toxic worldview is to allow them to be part of communities that aren't that cult or that worldview. The more insular you force them to be, the more insular they will become. Within reason of course and especially with cults, within what's safe for everyone.

Sorry, long ass post. Tl:dr, they don't necessarily believe the earth is flat, they believe that they believe it because that allows them to join a social group.