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.

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.

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.