In my world at least, mostly because Gods aren't all powerful beings right from the start. They become close to all powerful through training I suppose, but at the end of it anyone could become God if they have a pure heart and enough power, as well as a will to protect and look after the world.

[Bulma is all fine with this actually, because her scientist part has issues accepting that humanity was just created by a deity. She can believe in magic and such but not even Shenron can just go and create something out of nothing.]