Mods can ban users and delete posts, however to fully remove a user and/or their posts from the database an Admin is required.
Some of the topics you created could be of user to other users who are having similar issues. If your posts were deleted then topic such as this one
here would lose its logical flow making e.g. my posts seem redundant and out-of-place. If you really want you account deleted I would suggest the posts be left in place.
Cheers, Arite.