Video game movies combine everything terrible about both.
Hollywood doesn't make good video game movies for the same reason General Motors doesn't make motorized unicycles: they're stupid ideas, there's very little money in them and they aren't very good at making their regular product to begin with. When the reason that a product exists in the first place boils down to, "Out of ideas," the chances of producing a high quality product aren't great.
As far as movies are concerned there are two types of video game plot
1. The Crazy Ones
If you weren't distracted by playing the game you'd realize most video game plots are insane. And when riding blue dinosaurs or beating up schoolgirls in an airforce bases is all that's stopping you noticing real madness, you make the Joker look like a camp counsellor. Eating flowers to defend yourself from spiked eggs thrown by a cloud isn't a scene--it's when your therapist decides to buy a yacht. Hence Hollywood takes some liberties.
2. The Perfect Ones
Some video games have plots that would be impossible to screw up as movies. Unfortunately, Hollywood screenwriters take that as a challenge, which is why we had a House of the Dead without a house, and The Legend of Chun-Li (in the game a kung-fu expert Interpol agent beating up a psychically powered crime lord) became a classical pianist versus an Irish estate agent.
Last year the entire video game industry was worth 22-billion dollars. The Dark Knight grossed a billion by itself. Expecting a movie to be faithful to the source is like expecting Milla Jovovich to remain faithful to Level 70 Blood Elf (which will happen as soon as those idiots accept our script). They've already got the original fans by putting the name on the poster, and if they can convince another million idiots to turn up by adding a retarded romantic subplot, Tara Reid as a scientist or Bob Hoskins then they'll do it and still sleep like babies at night. Remember: In box office-ese, "seeing the movie then ranting about it forever online" translates as "seeing the movie."