I think one reason is because people are rarely content to accept that what is is all there is. They think the observable world is somehow mundane, and the world cannot be so boring as to be without supernatural battles between good and evil (the only place these two concepts are clearly defined) that somehow revolve around humanity.
People in this situation are usually ignorant of science, which in my opinion is far more intriguing, beautiful and emotionally powerful than any religion. Once you accept that space and time warp in response to the presence of matter, that every particle is also a wave, that space-time itself is expanding in the wake of the decay of a mass-energy plasma, and that information can be transmitted faster if it goes via another universe, I think the idea of someone speaking the universe into existence is juvenile and mundane.