Step back even further -- why is there "an existence" at all? It's a somewhat moot question, given that there is "an existence."
But just try answer the question "why is there existence?" You'll find yourself stumbling over all manner of mind-bending issues -- such as "how did existence get there at all?" One thing seems clear: "existence" is independent of time -- it's literally eternal (I confess I do not understand the full implications of eternity). Perhaps there is no satisfactory answer to "why" there is "an existence."
The question of "why" there might be a God is along the same lines. If there is a God, then He would share, in some sense, the properties of "eternal existence," in which case the question of "why" He exists at all seems rather moot.