Because belief is intrinsic to humanity even if we don’t believe in religion.
I believe in a lot of human concepts, including kindness, altruism, democracy and humanism. They are all still effectively made up human ideas.
I also believe when I sit down that the chair below me really exists but I cannot truly trust my own senses 100% either. So effectively I “believe” what my sensory organs and brain interpretation tell me, but the reality is the brain and its interpretations can be wrong.
Look at the USA, the founders of the nation are often treated with a reverence akin to that of religious figures.
People have all kinds of delusions. People worship all kinds of weird things. Religion is just one of many.
Finally, someone like Ayn Rand shows that a human can have pretty reprehensible and hypocritical beliefs even if they are an atheist. She promoted bullshit “great men” theories of humanity and argued that selfishness could be used for good.
She also died penniless and on government benefits while spending her whole life preaching against things like government benefits.
People are deeply irrational even without religion.