Hitchcock's Psycho Traier

There's a short clip of Hitchcock's Psycho trailer in The Pervert's Guide To Cinema, in which Hitchcock gives the viewers a tour of the Bates Motel and Mother's house with some hints on the plot. I found the complete trailer on YouTube -

Compared to a conventional trailer which presents a collage of the actual scenes, the Hitchcockian one is pretty clever and playful, though a bit too long. So the question is: Why does a movie trailer have to contain actual movie? Why can't we have something more creative??