Why Do Atheists Criticize People's Beliefs?

[Valyza1]

Belief is simply the psychological state in which one accepts a premise or proposition to be true.

Agreed, but it's not as if people just accept a premise or proposition as true right out of the gate, which is my point. There's usually some truth or intuition which reels them in and makes the untruths sound plausible. That's actually a principle confidence tricksters use quite often.

When ancient Greeks believed in Zeus, Andromeda, Poseidon, etc, was there a kernel of truth to their beliefs?

If any god exists, then yes. At best, people have directly experienced divine revelation. At worst, no one knows for sure. There can't be a preponderance of evidence for or against the existence of a god, so there isn't a way to determine the odds.

Flat earth, geocentrism, 6000 year old earth, Ginn, garden fairies, all beliefs people hold, either in the past or currently, have no kernel of truth.

Some of those conclusions are likely based erroneously on reasoning from biblical verses, which are conceivably subjective truths, rather than objective truths. Like poetry.

(Incidentally, I don't know of anyone in the last 500 years who believes the earth is flat on a macro level, either).

https://theflatearthsociety.org/home/

Also search on youtube for flat Earth. CoolHardLogic has a great series on flattards, as he calls them.

I would assume that's the fringe, yes?

But you are aware that people fervently believe completely contradictory things all the time. Is the kernel of truth you are referring to then the common denominator of all of those? You won't be left with much, if anything.

Either a common denominator or the kernel of truth is different because it's subjective truth. One guy believes that women are generally insane, because that's been true in his experience, and another guy believes that women are generally calm,because it's been true in his.