I believe the big difference between game demos and movies is that games are both longer than movies and much more expensive for the consumer. If you lost $8 on a movie on a Friday night, it might not be as big of a deal as losing $60+ on a game. Even if the movie sucked, you could still have a decent time overall (making the movie a small part of a larger evening). It's a relatively quick experience. Many people buy games hoping they'll provide much more than 2 hours worth of entertainment. If the game is terrible, paying that larger price seems like even more of a loss.
Sometimes I'll see a movie if the trailer is bad. I'll almost never buy a game if I didn't enjoy the demo. The more expensive the entertainment, the more critical people are about it and the more stingy they are with their money, at least in my experience.