Going to a movie is essentially a one-time thing. For most people, you go to a movie once, and most likely, you won't see that movie again until it comes out on DVD. Unless what the movie critic says is the deciding factor in whether or not you go to see a particular movie, you go to the movies and don't find out if it's good or bad until after you've already seen the movie -- and the movie theater already has your money. The average moviegoer goes to see the movie and figures that if it turns out to be a dud, they're only out a few bucks.

That's especially true for a film's first week. A bad movie that's a sequel to a popular movie, or that had a huge advertising budget, might gross a lot in the first week simply because everybody thinks they have to see it, not knowing that it's a bad movie.