They remain on for one simple reason...
They're dirt cheap to produce and somehow manage to find an audience. Producing an entire season of a one hour "reality" show can cost less than a single episode of a hit sitcom. For the last year or so of it's run, each of the 6 characters on "Friends" got a million dollars an episode. Add in the cost of writers, etc. and the cost gets real big real fast. "Reality" shows have few if any writers, no talent, and a prize that is often paid out over a period of years. As much as they suck, the bean counters (and a lot of people) love them. One can only hope this passes soon.