LOL can't believe people are still asking this question on 2017. Literally every ancient civilization knew that the earth is a globe. The bible thumpers are a disgrace. They manage to distort Every Single Thing about everything. In their minds, everything is about demons and lies. Can't wait for the truth to be revealed, so their evil empire can disappear forever.
My feeling is: It's not about the truth. It's about them being recognized through the effort they put into their conviction. Most of them aren't crazy enough to actually believe it. Many of them believe it because it's a controversial subject - i.e. it causes people to get angry, not that it's actually of any value - and they want to feel like they know the truth, even if there's a ton of evidence against their belief three seconds of googling away.