Pretty sure flat earth is a parody right? No one actually believes that do they? It’s like the flying spaghetti monster or the church of the subgenius.
Regular people aren't as interested in space exploration as nerds tend to be, so where we could point to dozens of things that wouldn't make sense or would be much more complicated if Earth was flat, their only basis for this belief is "because the society (school, in particular) says so". Within this group of people, I'm sure there's a subset for which that basis is not enough against their belief that society/scientists/rich are out to get them.
> "I encourage you to go to a flat earth meeting or conference."
Selection bias. Of course the trolls wouldn't bother going to such a meeting. The proportion of true believers at a conference says nothing about the proportion of true believers in the general public.
One of my old friends got sucked down the rabbit hole of YouTube conspiracies. Someone who was once fairly normal now believes in all kinds of crazy things, including flat Earth. It's definitely a growing belief.
This isn't really a conspiracy, this is the "cognitive infiltration" technique advocated by Cass Sunstein. Basically, even if the government doesn't like conspiracy theories (especially ones around purported government action/inaction like 9/11 truthers, JFK assassination, etc) they can't ban it or punish people due to the First Amendment. Therefore the best approach for discrediting them is infiltrating the movements and pushing them to even more bizarre extremes, so that "normies" will not become interested in what they're peddling.
Some people believe it. Some people are trolling. Some people were trolling and realized all the power and influence they have by continuing to troll.
The primary common aspect is distrust of the word government.
There are very inquisitive and scientific people in the Flat Earth community that simply distrust particular sources, but simultaneously jump to conclusions that are opposite of that source. So they are interested in proving or disproving the same thing, but they are handicapped because they can't simply reproduce existing scientific research, they have to categorically reject that too simply because the state was involved in funding or agreeing with the research.
Eventually they do run interesting experiments that undermine their flat earth theories, but they are still trying to prove the earth is flat, instead of trying to prove the shape of the earth and accepting where-ever that leads them. This is distinct of how science is supposed to be done, but some of their experiments are clever.