God's not showing up till the final curtain.
God's already shown up and was featured in several episodes. He even made a (very) indirect appearance in this one.
God doesn't want the gates to hell closed.
Otherwise they'd be closed.
God clearly sees humanity as some kind of experiment. And considering that, as far as we know, only humans get to be prophets and only prophets get to read the tablets, they were clearly created for humanity to find, translate, and use. It also fits in with the whole "free will" thing.
God also prefers a hands-off approach to creation, but maybe he slipped these in like cheat codes in a video game. Just like the novels he wrote and the small nuggets of help he's provided indirectly over the years.
Why is God doing it that way? Who knows. Maybe he just can't bear to destroy his original creations himself, so he gave humans the ability to protect themselves from them. Maybe he's just a bored old prick toying with life like rats in a maze purely for entertainment.