As a discussion board, the removal of your posts would destroy the conversations you participated in, so the posts would not be automatically removed. The lack of a "delete my account" option helps remind folks their old posts are not going away unless you manually delete each one individually.

The purpose is to make sure the posts are not deleted or replaced.

Most people are not going to to go through and manually delete hundreds or thousands of posts.

You're basically asking "Why doesn't DU allow me to do something that the Admins intentionally and deliberately made a decision to not allow me to do?"

I think the answer to that is self-evident. You can in some sense self-delete...go to your settings screen, change your password to gibberish that you won't remember, change your email address to no@nomail.com and log out. You will never be able to log-in again. Your posts will remain because there is an intentional decision made by admins that they must remain for the site to function.

If large numbers of posts on the site were to start disappearing when people deleted their accounts or were banned, the site would be rendered unreadable quickly...so they make it as hard as possible for people to remove large numbers of posts in an automated fashion.

You're complaining about an intentional decision and wondering why they don't allow you to do something they clearly made a conscious decision not to allow for a very obvious reason.

If you don't like it and want to leave, spend the next 2 hours individually deleting all your posts, then log-out after making it impossible for you to log back in.