That way if someone has the same question or follow up questions they can go back and contribute something instead of having multiple threads on the same thing. iPods for instance.
Also its incredibly tacky to have some mod not even involved in the discussion to arbitrarily decide that the thread is over. Especially if it was staying on topic. Threads will die if there is no more apparent reason for their existance, thats the point and nature of a forum. If someone still has something to say, it wont die. Marvelous, eh?