What? God? "God did it." is not science. Science is natural. Science is explainable. God is neither. Still no reason why.
Excuse me for being blunt, but what the hell do you mean "why?"
You think the earth is a sphere, so WHY is that?
You say science is natural, so WHY is that?
You say science is explainable, so WHY is that?
You say earth
isn't special, isn't the center of the universe, isn't the only thing that can support life, so WHY NOT?
You can keep saying why why why to everything until the end of time, and to each explanation you can say "why" again.
The earth is what it is, and that is why enough for me.