If God exists, why is there some Being rather than no Being?
Put another way, with regard to the existence of contingent beings (“Why is there something rather than nothing?”), God presumably functions as the necessary and sufficient cause for those contingent beings. But on the principle of sufficient reason, God has no reason for being Herself.
So the answer to the question, “Why is there some Being rather than no Being?” has no answer. If God exists, God is a brute fact.
Thus wherever there are contingent beings, THEN it would appear that a singular, unified, necessary Being must exist to end the infinite regress of causes. There must be a necessary cause of some sort. But that necessary Being itself has no reason to exist prior to the creation of those contingent beings. There could have been no Being rather than that one Being.
Therefore, the existence of God does not square with the principle of sufficient reason. God is either a brute fact without a sufficient cause Herself, or She simply does not exist.
But then what is the necessary cause for contingent beings?