It’s actually much more technical than theoretical. When you delete an account on a website, that is being kept for a little while longer, it merely has field in the database that gets updated. (often with a removal date as well for the automatic removal after x amount of days). This field needs to be checked everywhere the account is used. And account recovery is mostly a part where this is forgotten, or possibly not even wanted.
And to claim this as fact, I just realized that the website I work on allows recovering of banned accounts. (Removed accounts are completely removed though because we don’t need to retain any data).
This is the only valid reason. But even then this could be stated so that the user is fully aware.
Keeping the records for a little while longer is actually implied to be known. It’s in their privacy policy, and is legal.
Whether or not services should make this easier to know exactly what is happening I definitely agree. Personally I think post history without user identifiable data should also be removed, but this is even less common practice (and is why tools exist to delete all your reddit posts for example).