Closing and downvotes represent two different kinds of community handling of the site. While oft similar, they are actually different axes that are traversed.
Downvotes represent quality and clarity. Or rather, the lack thereof. People downvote a post because the post isn't useful to have on the site, or because it's terribly unclear what the post is trying to present. So you would downvote things that are probably need fixing in order to be a good post in terms of quality.
Closing, on the other hand, addresses feasibility and scope. We close things to indicate that they do not belong on the site in their current state. Some of these will never outgrow this state (thing that are off-topic for the site's scope), while others can be fixed from that state (unclear what is asked, etc.). Then there's duplicate closures which are sort of an alien third plane that exists both parallel and perpendicular to this one... so let's put those aside. Putting that aside, the general eventual goal of closure is to exit the state of closure - either the post is fixed and no longer needs to sit closed, or the post is not fixed and so it becomes deleted.
Closing and downvotes intersect when, say, you deal with a post that is unclear and needs heavy fixing to be viable on the site - it may be closed and downvoted while it sits in the broken state. But that's pretty much the only intersection point, and in this point both usually are an option simultaneously, not as alternatives to each other. But if the post isn't low quality, if it remains closeable there's no need to downvote.
Stack Overflow has a bit of a history, though. In being as old a site as it has been, there's a lot of change in what the community wishes to keep and maintain, and in what the community enforces for quality and scope guidelines. There's a lot of rich history that exists on SO, though, that by today's standards wouldn't be allowed to be asked. We are faced with two choices in this case.
- We follow our traditional methodology of close -> delete, in which we then eliminate these so that they are no longer on our site. While this is methodologically sound since the content does not fit what we would like to keep on the site, this would also destroy content that is, most often, quite useful to have.
- We leave the items around but closed. This breaks our general protocol but serves our mission as a network, which is to make the internet better. There's helpful content contained even if it doesn't match our model - removing it means there's nowhere to host it, and until such time that there is a place that hosts it, we are said hosts.
It's not an enjoyable choice to make. But we picked the second route. They remain closed because they do not fit, but we'd be better served for now to keep them around than to destroy them outright.
The younger sites avoid this by keeping stricter terms - we avoid needing to host "old but useful" stuff on younger sites by avoiding the accumulation of useful-but-not-accepted content to begin with. So they tend to stick to "closed -> deleted" as a pathway.