Speaking from personal experience, substantial edits to existing answers are usually rejected because
- the reviewer may not always be an expert in the topic discussed
- even if they are, some are concerned that the edit will be mistaken as part of the original post
- there is a chance it may negatively impact the author's reputation
the last two are usually the key that prevents outdated posts from being edited, at least by people with less than 2000 rep, like me.
But for posts made by deleted accounts, it no longer matters who made this post. If they deleted their account, it is implied that they lose the ability to edit the post as the original author, to receive notification about edits on the post, or to track what they have posted in the past. Neither will these posts be associated to their reputation (on SE or IRL).
It is, in my opinion, effectively the same as Community Wiki, only with a higher reputation requirement to edit. But in this case, enforcing a higher rep requirement on editing only prevents collaboration.
So, why not convert them to community wikis?
P.S. I have read why auto-converting to community wiki was disabled, but it doesn't mention anything about deleted accounts.
Edit: As @rene pointed out in his comment, converting questions to CW has the unintended effect of converting answers (including existing answers) to CW as well. So I'm going to limit this proposal to answers only.