To keep a record.
Since IP is hardly any use in identifyining misbehaving individuals, our moderators have developed Sherlock Holmes quality skills in detecting them by usage/writing patterns. Reoccurring issues, themes, slang words, typos…
They have caught a dozen offenders by comparing old and new posts. Having the user handle in the database helps one to remember the case. Some are so stupid that they use similar user handle compositioning with their new accounts.
We always suspend for 9999 days and never delete.