I'd like to point out two more thing things.
(1) Deleting questions is basically an invitation to ask them — and close them — again. The record of a closed question (for whatever reason) constitutes a form of precedent that allows us to help educate and guide new users. (Whether or not we do this effectively is an entirely different question.)
IMO, questions should only be deleted when it's obvious they don't belong on this site (such as programming questions that should have been on SO and, for whatever reason, appear here. It's happened several times in the last year.)
(2) Curiously, seeing a large percentage of closed questions with a specific tag shouldn't (in my mind) have led to the question, why aren't we cleaning up after ourselves? Rather, it should have led to the question, what's wrong with this tag that questions associated with it get closed so frequently? It would suggest...
- that the tag is badly defined,
- that the tag is inappropriate for this site,
- or that we are not helping users (especially new users) to understand the tag.
You didn't mention what that tag was. It might make more sense to post a Meta question about the tag so we can come to a consensus of how to better deal with the tag and reduce the number of closed associated questions.
A specifically relevant meta question: