People aren’t rational beings - we mostly operate on emotions, fueled by chemical reactions to certain events and sensations and experiences.
Most flat-earthers probably don’t care about any of the “facts” or “explanations” they hear or spread or study or come up with - first and foremost, it’s a community to them, a place where they feel like they belong and such. For their own reasons, they allow the obviously positive emotions they experience there to outweigh any of the absurd they may honestly recognize internally, but never admit or voice out or truly give in to.
I think I’ve seen several somewhat lengthy videos on YouTube on the matter, explaining how and why that happens. It’s a mechanism similar to other conspiracy theories and communities around them, as well as various cults - vulnerable, susceptible people are the ones to usually to end up in these because they’re reeled in one way or the other.
I’m not saying the theory isn’t nonsense, of course; only that the theory itself is probably only a facade for a way for some people to experience connection with others, a sense of belonging, some shared activities, something along these lines. That’s why you shouldn’t be surprised that their numbers grow or that they can easily ignore facts and science - it simply isn’t about facts or science, but emotions and feelings.