Is the movie industry actually... an industry?

I was just looking at the news and I noticed that Jackass 3D had taken $50m at the US box office in a weekend. It cost $20m to make.

The simple rule is that a movie is a hit if the box office gross is 3 times the budget, so my guess is that they'll be a certified hit by next weekend.

Which begs the question... why aren't more people making Jackass-type movies? Or if you look at certain genres of movies, they seem to make money hand-over-fist. Critics might like Martin Scorsese and Spike Lee, but they actually never make much money. The movies that make the best returns are horror and teenage movies. You don't need an expensive cast for something like American Pie, yet there's a huge teenage audience for it.

My theory is that a lot of movies get backed by people who are drawn into the glamour of Hollywood, rather than on making a return. Thoughts?