To summarise this post from the network blog, closed questions in great numbers add noise, are toxic, and reduce confidence in the system. (Exact duplicates are excepted as they arguably do add value.)
We already automatically delete questions that the community have allowed to belong here.
So why aren't we automatically deleting closed questions that the community has stated don't belong here?
Closing a zero-voted question drops its value to -1 and may make it eligible for deletion, but there are plenty of upvoted closed questions that don't belong either.