I suppose one of the possible reasons would be because such function could be abused in various ways. A quick example:
A user posts something which appears on the music that may not be posted for legal reasons (
Thread 49829) thread, the thread stays open for a while, before its creator opts to delete it.->Due to a set of circumstances, staff members are none the wiser about this.->User safely avoids punishment.
Now that I think about it, some goddamn Shriners who thought they were extra clever made use of "User Groups" to share forbidden material.
At any rate, if you want a thread deleted, you can request for its deletion.