It is done by design. (To keep deleted records.) There are many reasons, one of the most important is to keep track of who/what was on your server for legal purposes to help protect yourself. It is also there to prevent spammers from re-registering, etc. There is no cron created to remove these entries.
I suppose you could make a cron to do this, but it is not recommended.
I suppose you could make a cron to do this, but it is not recommended.