My theory: Most of the people who claim to believe in the Flat Earth do not, and that this "Flat Earth Society" is being used as a tool to inspire people to question the things that they are told. To perform experiments to practice the scientific method, and try and prove the things we take for granted for themselves. I consider this a noble cause, and if there is any truth to my hypothesis I applaud all involved.
The alternative is rather silly, considering the sheer number of people that would have to be involved in such a conspiracy.
-Starting first with the over 450 people from all over the world (in countries that are somehow all collaborating together on this one huge cover-up) who have allegedly been to space, ranging from pilots, to scientists, to people in varying other positions.
-Moving down to the even larger group of people who actually put together the space program, and spend large chunks of their lives collecting and analyzing (sorry, faking) data. After all, if this conspiracy is to make money off of a fake space program, the people who would be spending all that funding on actual technology (including people not associated with NASA, but who still wish to utilize the space program) have to be told about it.
Take for example Dr. Samuel T. Durrance who, along with a large group of researchers and technicians at various levels, decided to build a UV telescope that would be used on the orbiter. Dr. Durrance was one of two scientists who would be sent up with it to operate the device. Did they spend all the trouble lugging that thing across the center and into the (fake) orbiter attached to a cord to keep the vacuums running so that it would be in optimal working condition, and then tell him "Oh, sorry, there is no space program. There is no way you can use that machine you, and the students and workers with you, spent years of your lives on. However, you must still hide out here and pretend to be in space, then go back to them and tell them you did, and write an emotional note about your experiences with accompanying pictures and videos we will fake for you and tell everyone you took."
It's not just space sciences either. I know researchers currently developing manufacturing processes that will be used in space labs. Specifically, to make solar cells in zero gravity, where it is more feasible to construct the crystal structures required to boost energy efficiency.
And it's not like all scientists are in on the conspiracy too, right?
-It continues to branch out further from there, probably including those in aviation who would be in a position to observe the improper distances resulting from a flat earth.