Who ever come up with flat earth is retarded tbh
Yet all over the world there are people who are not scientists and mathematicians. They go outside and look around them. They see that the earth is relatively flat with a sky-dome over their heads. That is what anyone can see. So, they are not retarded. They are simply ignorant of things, just as you and I are ignorant of other things.
Well... You don't have to be any kind of scientist to look at pictures/videos/livestreams of earth. Besides people figured out the earth wasn't flat before we even had photographic evidence, looking at boats over the horizon or many other easy experiments or observations would prove the earth is not flat.
You and I can do this. You and I have done it. But there are many people on earth who haven't done it, and there are many others who don't have the available technology, or the motivation to use technology to do it. They aren't necessarily retarded simply because they don't hunt down the facts to prove to themselves something that they aren't interested in, or that makes no difference to them if they know it or not. When they look outside, they see a relatively flat earth.