Joke
They do it as a sort of intellectual exercise or battle of wits. I reckon that most don't even believe it. What gets them every time is that they have no argument against and no explanation for the gyroscope tilting as you fly around the planet. The lighthouses that sink as you sail away from them are also quite difficult for them to explain.These flat earthers do seem to have a lot of support.
Apparently they have members all around the globe.
That's just spacetime curvature. It's not that they don't know, they just don't think about it.no explanation for the gyroscope tilting as you fly around the planet.