I am pretty sure that only moderators can delete posts and threads. But when they are deleted it affects your Bench Bucks which is why this is not being done. I am not 100% sure on this but I think that is the reason.
Posts are not deleted for cleanup purposes, except that old posts are archived after 12 months. Posts affect the number of Bench Bucks you accumulate and they also are used as a measurement to be awarded the Bench Hall of Fame.