I've met some genuine Flat Earthers - it's a disturbing experience to meet people who are that far gone. At first I thought that they were pulling my leg in jest, because it seemed to me nobody could be quite that stupid. Then, after talking to them, I realized that yes indeed, there are people that stupid in the world.
When I asked them about all of the photos taken of the Earth from space, they said that the photos were just CGI fakes. I pointed out that in 1969, there was no CGI, to no avail.
Then I pointed out that with a hobbyist's telescope you could see that the moon and various planets are spherical, and they said that the Earth is the exception - it's flat even if the other planets are spherical. When I tried to point out that the same spherically symmetric gravitational processes that caused space dust to accrete into the other planets formed the Earth in the same way, they said something like "the Earth is just different" again.
You can't fix brainwashing and a you can't fix stupid.
same with people who think the moon landings were fake
or that joe biden actually won
Intelligent people can easily believe things which seem “stupid” to people who believe something else.
I knew a very intelligent man who claimed if all the ice in Greenland melted, it could not possibly raise the level of the oceans as much as was claimed, because Greenland was just a small island.
I looked up the numbers, did the math, and convinced him he was wrong and the math showed the oceans could rise that much. He had never actually done the rather simple math. He just assumed “those people” were lying to him because they had lied to him about many other things.
I also explained one of the real lies was that the melting of the Greenland Ice sheet would happen quickly. That helped.
Some people are extremely intelligent about many things, and extremely ignorant about many others.
We are all ignorant, just about different things.