It's not satire. These people are truly messed up. Usually, they're ultra-religious or they were already conspiratorially-minded (space aliens killed JFK, etc.).

It's just another cult; People who couldn't fit into society eventually clump together and use these beliefs as a filter as an intellectual barrier to join their society. Instead of a physical wall, you have to believe that the Earth is flat. And, in order to stay in the cult, you have to continue to demonstrate that belief or get kicked out of the cult, hence the constant promotion. And they don't want to get kicked out of the cult because that's their only social network that they have so it becomes an endless cycle.

It's just like Trumptards.