Closed questions are deleted, at some point.
But not immediately, there is always a chance someone will step up and salvage a closed question. There is indeed a very large number of closed questions, mostly due to historical reasons. Mark Trapp's answer on How to reconcile guidelines, community opinion and moderation gives an excellent outline of those reasons.
As ChrisF mentions:
It takes time to work through all the closed questions, working out which are salvageable and which need to be deleted.
People complain bitterly if highly upvoted closed questions are deleted.
We have a gigantic clean up effort going on for at least six months, involving career related questions. This and other clean ups have generated a fair share of Meta drama, so right now we are focused on better organizing clean up efforts, and part of that effort is for community consensus to be crystal clear.
The path to deletion is simple, every closed question will be deleted at some point. But that doesn't mean that every closed question can't be re-opened, so we must allow for some time before a question is deleted. We may be a bit slower in deleting questions than the trilogy crowd, but it's only a matter of pace and not one of going against common SE practices.
- We have a lot to clean up,
- Let's worry about deletions after that.
Searching for closed questions doesn't distinguish between close reasons. Duplicates, for example are not prime candidates for deletion, for SEO reasons.
I've build a SEDE query that doesn't count duplicates, and the close total is fairly smaller: 1936. The current SEDE data are valid up to Dec 22, but I don't think we closed a lot of questions since then (?!).