Once you internalize that flat-Earther-ism isn’t about the Earth being flat you realize that rational arguments are pointless.
To expand on that, it’s about community and finding people who share your interests. The movie Behind The Curve explores this idea and it’s quite revealing.
A bunch of flat-earthers went to Antarctica to see if the midnight sun was real. Turns out it was.
Jeran from Behind The Curve was one of the ones to flip, and since then, he's been making videos on how the earth is actually round.
He has a lot of thoughts on what it actually takes to convince other flat-earthers. I found it somewhat interesting: https://www.youtube.com/watch?v=1grMf17PeEk
It's since being replaced by similar isms like climate change hoax-ism. Very similar way of arguing, dealing with contradicting evidence and seeing a conspiracy whenever a large body of scientists has a consensus.
Unfortunately, the climate change deniers in all their forms have made it much further by having support in politics and having a real impact on people's lives. In contrast to flat earthers.
Just the mere fact that my post here could be interpreted as political (which it really isn't) is evidence of this.
It's more about discrediting conspiracy theories to shift the Overton window so the real ones with the flavor of 'the government is spying on you' also seems crazy to most people.
And the ego boost of it all - being one of the special few who sees "the truth" that others are too brainwashed/dumb/whatever to see. Makes one feel quite important.