There's really only 3 possible explanations for the Flat Earth society. I'll list them in order of my opinion of likelihood.

1.These guys are master trolls. They've maintained the gag for a fairly long time with very little wavering. A lot of even this website is proof that people just LOVE to get a rise from people on the internet, and few things tend to get people worked up like flat earth talk.

2. A lot of the stuff they talk about rely heavily on forming opinions based on one's own personal observations which ins't an ENTIRELY crazy position. Why shouldn't I believe something if I've never experienced it personally? It makes sense metaphysically. The ridiculousness of the claim just helps make the point. I mean, really think about it, how many of us have actual first hand unequivocal experience that the world is round? Admittedly, most rational people can acknowledge that a mass of third party evidence IS acceptable to take a stand on something.

3. These guys are just bat poop crazy.