Im not sure but i would like to know also.
Originally Posted by GH0
It has been suggested in the past many times in the past.
The option was never really accepted as a good idea. As mainly, the user that would like to close his or her thread is able to report the main post saying "Please close at the request of the OP" and the thread will be closed.
There is also a bit of room for exploitation here. When someone doesn't like the replies they are getting or doesn't agree and they want to close the thread and start a new one.