Because religion as it exists today is nothing like the religion of a hundred years ago, and nothing like the religion of prehistoric peoples. Modern peoples, especially atheists, and especially on Quora, make the mistake of thinking of religion as a philosophical viewpoint that anyone can just discard because they were able to discard it.
Religion is far, far more than that. It’s how businessmen in the 1500s recognized who was worth trusting with a job or a contract and who wasn’t. It’s how entire societies were organized.
How it evolves is tremendously fascinating. You come away with a completely different perspective on Christianity if you study the actual environment the Judeans lived in back in those days. Modern Christianity has about as much to do with the real Jesus, assuming he actually lived, as General Tso’s chicken does with China. But to discover how it became that way is to make a sweeping study of Roman and medieval cultures and the limitations of social technologies of the day. Christians, as far as I’ve been able to tell, just try not to think about it.
Religion built the pyramids, religion is arguably responsible for civilization itself. The story of humanity is inseparable from religion, in fact one can argue that you cannot really arrive at a true essence of humanity without tossing in some religion for flavor.
My go-to book to recommend to anyone looking to understand religion is Fields of Blood.