I personally believe there are two reasons. Either they are attention seeking idiots, or they are mentally ill.

Here's my standard answer to any question about Flat Earthers  

Summary: there's only a handful of Flat Earthers, they don't have a common senate and shared belief. Their 'belief' is beyond dumb. To entertain them is at best pandering to attention seeking idiots, and at worse mocking the mentally ill.  

Here's my standard response to Flat Earther questions.

Point one: they don't really exist, outside the Internet curiosity in them.  

I worked for years as an Earth Scientist in that time I recieved emails from creationists and Climate change deniers on an almost daily basis. I never heard from Flat Earthers. Likewise I've never seen a Flat Earther answer a question like this on reddit. They don't exist in any significant numbers. Of course there are some random nutters but there's not enough of them for there to be any consensus or single theory they subscribe to.  

Point two: suggesting Earth is flat is beyond dumb.  

Let's use proof-by-contradiction to disprove their 'theory' by assuming they are correct.  

This means that the following people are in on the conspiracy: anyone with an Earthsciences, Astronomy, Geology, or Oceanography degree; all of acedemia; anyone in global navigation such as airtraffic controllers, aeroplane pilots, ships navigators; everyone working in space exploration; and of course anyone in government. Mass and masses of scientific data would have to be faked. Do you know how much time and effort it would take to produce near real time seismic data from a global seismic array? Pictures of Earth from Space would all have to be faked. And everyone involved in that conspiracy would have to remain quite. And this would have to have been going on for thousands of years. 

And they would all have to be sworn to silence for a conspiracy that served absolutely no purpose.  

Point three: don't give them the time of day.  

So there's a hand full of nutters writing blogs for motives that range from stupidly and paranoia, to attention seeking. Why are we even entertaining them? There's proper science that is far more worthy of our time. Sure it's amusing to try and work out what's going on in their heads. Mocking the insane beliefs of the mentally ill is the modern equivalent of visiting a freak show.