I think you have the causation backwards: we have people thinking the world is flat because they can access the sum total of human knowledge, both true and false. There’s so much available, with similar production values, that going down brainwashing rabbit holes like flat earth, anti-vax, and more is a lot easier than it has ever been before.
I've been slowly coming to the realization that a large percentage of people are just counter-cultural, be they smart or stupid. We think of the term in 1960s hippy movements, but some people want or need to believe there is a conspiracy or that everybody is wrong and they have some truth to believe in. Ignoring the people profiting off of these movements, I'd be curious to know if they just crave some kind of intellectual stimulation, are looking for an alternative to religion, or if it's something else.
Let’s be real, some people are going to believe absurd things even if you strap them in a chair Clockwork Orange style and force them to consume your favorite propaganda 24/7.
There is no way to “align” human brains to your preferences. The Soviets tried it, the Chinese tried it, the Americans tried it. Nobody succeeded. The best you can do is attempt to sway the masses, but you’d better rely on positive messaging, because mass culture’s failure modes are even scarier than small subcultures.
Attempting to stamp out competing worldviews leads a certain kind of (relatively common) person to dig even harder for forbidden knowledge. If you’re not careful this will lead people directly to the arms of your geopolitical enemies, as it’s not possible to fully stamp out their narratives—they have a big budget!