Basically, the earth is a flat disc protected by an invisible barrier called “the firmament,” a dome-like feature that also happens to be referenced in the Old Testament so you know it’s real. Copernicus, of course, fucked everything up by introducing his theory of heliocentrism. Flat Earthers acknowledge that while Copernicus’s math was right, he lacked the technology necessary to definitively prove his theory. Still, the idea took hold, governments started preaching it, and a centuries-long lie was born.
Then as soon as Russia and U.S. managed to get rockets in the sky, they realized their error and quickly set about protecting the shameful secret at all costs. The Antarctic Treaty went into place in 1961, making it in “off-limits” to any wandering eyes.
It’s worth noting here that there’s also something called the “ice wall,” which is the towering expanse of maybe absolute zero, pitch black ice barriers at the edge of Antartica that may or may not go on forever and that no human could ever possibly explore.
Gravity is something that might also have thrown a wrench in the Flat Earther’s personal Truth—if it weren’t for something called universal acceleration, that is. You see, the world is constantly accelerating upwards, forcing everything else “down.”
What’s more, the extra land that comes along with a flat earth means more natural resources. And if word got out about the secret store of oil hiding at the edge of the Earth, any remaining notion of scarcity would be shattered.