I don't because I like to make as few assumptions as possible, and because I like simplicity.
One can go all nihilist and say that reality itself can't be proven, since observation itself is in question.
Okay then, but I'm sure both religious and non-religious people aren't nihilist. I'm sure we all don't believe that we're in some kind of matrix.
So we all assume the existence of the world, and that our observations are reliable. That's the axiom we all live off of in my opinion, our one big assumption.
From this axiom (that the world exists and our observations are reliable) we can do inductive and deductive reasoning. We can induce the fundamental rules and laws of our reality (inducing the law of gravity by seeing an apple fall, or dropping two objects of different mass from a building), and then use those laws to deduce things that we might not be able to see or observe. That's science in my opinion, and in my opinion everyone's a scientist. We all accept reality for what it is, we remember what reality is, and we make predictions based on that memory.
So the thing is, why don't we bring a god into our big picture?
A)To explain the unexplainable.
B) To explain the origin of reality
Simply saying that goddit isn't an explanation. In fact, that's just a more complicated answer. Here's how it looks in my mind:
?->God exists->Reality exists
vs
Reality exists
Even if we go with:
God always existed->Reality exists
vs
Reality exists
That's 2 things vs 1.
It's the same reason why I don't believe that little people live in my fridge, and hide when I open the door anymore. It's why I don't believe that everytime I go back to my car, someone may have replaced it with an exactly same looking car. It's simply unneeded to believe such a thing. Nothing changes, and it's just unnecessary.
Also, to me god is just the same as replacing one unknown with another unknown, as Thuderf00t says it.
?->God->Universe
?->Universe
Why do we need a middle man? Why believe in one extra, unnecessary thing?