Anything exists because it has to, since nothing does not and cannot. The end.
I was pondering the whole idea of nothing being an unstable state again last night. My problem with the idea was, as previously stated, that it seemed absurd to suggest that a state with the property of instability could actually exist and still be nothing.
But if we are instead saying that the perfect instability of nothing is what prevents it from existing, then in spite of the apparent contradiction we are not actually assigning the property of instability to nothing itself (which would make it something) but are instead simply stating that perfect instability is an impossibility because it must instantly be something else.
So, nothing is perfect instability, which can't actually exist. This means that Stegner's "unphysical state" isn't a "state" at all. He was instead simply bumping up against the impossibility of non-existence.
EDIT: This would still mean that his argument that the universe could have come from nothing is ridiculous. What he's really demonstrating is that it can't not exist.