You are absolutly on point about the improbability of a vast conspiracy of any kind succeeding. If more than one person knows about some plan to game a particular system or decieve the population of any society, someone will talk. The only way it can work is to seal all physical borders, then keep an iron fist on all communication both internal and external. It would take a society and government like North Korea to get away with telling your citizens that Americans live in massive tents, line up every day for coffee made from snow, and we’ve eaten all the birds in America because we are starving. All of these “facts” are contained in a film produced by the NK government and shown to the people of NK, who have no reason to doubt it because they’ve never seen, heard, or read anything to dispute or deny the info they are given. It’s the only way you can sell the idea that the million odd people in NK who die of starvation every year do so as a result of a conspiracy by the rest of the world to punish the only democratic republic on earth by refusing to engage in peaceful trade with them.
As for the odd mix, apparently mankind has been plagued by flat earth types since we emerged from the primordial ooze.
I recall an episode of Cosmos where Sagan was explaining the hypocrisies of Pythagoras and the ethical failings of Plato. He said the Pathagorean’s prefered that ordinary people be kept ignorant so they hid their knowledge of the dodecahedron and the square root of two, and that Plato’s arguments against the real world, experiments, and practicality where so compelling they emboldened his followers to eventually extinguish the light of science in Ionia and it stayed that way until the Renaissance, so apparently science elitists and science deniers have coexisted since the beginning.
Just one guy’s opinion, but we are well into a period where science denial is trending upward as fear drives us into ever more tribal clusters. Should we survive the coming madness, i expect historians on the other side will characterize flat earthers as quaint, harmless eccentrics when compared to some of the atrocities perpetrated by other believers in magic and mysticism.