G.T has it right, as nearly always.
By far, most threads are closed because they are duplicates of common subjects. We really prefer not to waste the effort of reinventing the wheel every time someone posts a thread without looking for the answer themselves. We'd rather that energy gets put into extending the existing discussion.
Other threads get closed because they are simply pointless and clutter up an already-crowded forum. And some threads are closed because they violate the rules of the forum, though those threads are most often simply moved to a non-public "recycling" board.