Why? Because if you are allowed to remove your account and the posts you have made, it may just leave the threads you have participated in, a complete mess of disconnected nonsense.
Thanks for the reply. Though it doesn't make sense. At all. Remove my account and change the name to "Account doesn't exist" or anything alike. Problem solved. Both happy.
For you to be able do that (the question you asked) would require that the developers of the forum software would have pre-empted that user decision and built in a means of accomplishing that task.
To have someone do that for you requires you to contact a forum admin and make the request via a personal message (NOT a profile post message)