We don't intend to. If you just leave, your account will sit there untouched. Think of it like Facebook except we don't steal your information and sell it for ads.
You'd have to ask some others, but I believe we don't delete accounts to prevent new users from signing up under old names and confusing others or besmirching reputations.
Also, the more names that are taken, the fewer that are left for spambots.