Maybe because those ancient societies had regular contact with more advanced beings from this planet or elsewhere in the galaxy, who explained this to them and refered to themselves as the creators or God's. Many ancient texts and art explain and depict this occurance.

The Cristian Bible, and every other religious text pretty much explains exactly what I said above, every ancient cultures believed this in one way or another. It's only in recent times that we have diverted from that point of view. And anybody today which truely has faith in any religion believes this now. It's only true atheists who have diverted from this idea which is actually a very small percentage of today's human population.

The vast majority of people throughout history or even today believe in creation or intelligent intervention regarding our existence.