Of the people who aren't just straight up trolling, I think a large percent just like to be "controversial" and hold contrarian views that might blow somebody's mind. The Flat Earth nonsense is a pretty low stakes one for most people. It's really stupid but it's not inherently offensive apart from the stupidity. It's not racist or xenophobic or anything. I think that's why it's gained so much traction.

Frankly the best way to have it fade away is stop giving them attention, 'coz both the trolls and the contrarians want attention.