You ask why answers aren't purged, but don't really provide any reasons they should be. Let's approach the question from that angle.
Why would deleted posts ever be purged from the system?
- Deleted questions are sometimes undeleted. Not possible if deletion was permanent.
- Sometimes moderators needs to make a judgement call about a user. The more information they have, the better. Having access to the user's deleted questions and answers can help the moderators make an informed decision.
- Since deleted posts rarely get viewed, they have little-to-no effect on the performance of the site.
- As explained in Why can I only delete 5 posts in a day? Is this legal? there is no legal reason why Stack Overflow would have to delete content that was legally posted to begin with.
I just don't see any significant reason to delete content permanently, and there are a couple of arguments for keeping content around forever.
So while I don't know which of these the Stack Exchange team considered, I think the current state of things makes sense.