I think Flat Earthism is a natural reaction to disillusionment with modern society, but specifically manifested amongst more academic members of the community wherein this disillusionment takes the form of a formalised rejection of modern scientific process.
Seriously though, I think it's mainly because it sets them apart, or makes them feel set apart. Either they've been told the Earth was flat and looked for reasons to believe it, or they're determined to believe in something other than modern science and, religion notwithstanding, they've latched onto this as their cause. I find it hard (but not impossible) to believe that someone looked at all available evidence and decided that the Earth was flat and simultaneously thought there must be a giant conspiracy covering it up. It's like the Apollo hoax believers. They like the fashionable anti-government stance and it gives them a community to be unique in. Conspiracy theories are, sadly, more fun than plain old real science. There's more of a what-if element to it, like there is with UFOs and stuff. Any thoughts on this? Any disagreements? (From REers)
I would suggest we started 'the round earth society' but it wouldn't get any serious members. Debating hard fact is a waste of peoples time!
I would like to add my thanks to ThEngineer and Dogplatter. As strange as you seem to me in your views, you do your society credit. Maybe just enough to stop Tom, Narcberry and Franc sinking it.