I was always under the understanding that an account deletion deletes information, I would have thought they could just recreate the user. But I could be mistaken
That is probably intentionally prevented by just soft-deleting the user so links from comments, posts,... to their author aren't lost. Not to mention potential security issues if you could just make a user that previously belonged to someone else. Honestly just isn't worth the headache to allow reuse of usernames.