Why are the users of IMDB (Internet Movie Database) so hard to please?

I'm a user of IMDB but I don't why the other users are so difficult to please. They usual overrate or underrate a movie. I mean on it's Top 250 list there are a lot of films I've never seen or heard of. Though it does have movies like Inception, The Lord of the Rings, Star Wars, the Pixar films, Avatar, Aliens and Terminator 1 and 2. But I wish more films I really love like Iron Man (1 and 2), The Chronicles of Narnia, The Karate Kid (1984) Predator and a few of the Harry Potter movies. The latest Harry Potter movie was on the list for a while but eventually was knocked off by the users.