What you need to understand is that for a minority of people who take truth, facts, reason and science very seriously, flat earth is infuriating. But for a lot of people who are just looking to have some fun, it turns out to be a lot of fun. The protests from the first group that you shouldn't joke about things like that, that it's wrong to be wrong, etc just sweeten the deal for the people having fun. It's trolling in the classic sense.
For most of them. Of course when you have people pretending to be idiots inevitably a few real idiots will join in, thinking themselves in good company. But for the most part flat earther arguments are constructed with obvious comedic intent that very serious people seem blind to.
Or you can think of it this way: being wrong is a taboo, and some people enjoy challenging that taboo.