Its already been said, but Ill add my two cents anyway.
I think the reason movies of game series suck is because of one of, or all three, of these reasons:
1. Games and Movies are completely different mediums.
This is a prety simple point, yes? But I think it might very well be the simplist point. Movies are meant to be watched, games are meant to be played.
Lets take a videogame made from a movie. Most will tell you that movie-games suck, primarly because they are made to cash in on the movie in question. But if you think about it, a movie is usually about 90-2 hours long, games are typically about 10 hours long (give or take for some). A lot of the problem with movie-games is that they are making a game about a movie, which means that have only 90 minutes of material to work with. So in many cases, they have to create content for those other 8 or so hours for gameplay.
Game-movies are the opposite problem, you instead have a game about 10 hours long, and you have to cut that down to about 90 minutes; which means in most cases removing alot of the gameplay sections which dont add any story. But with games, one of the most important parts is the gameplay, which does contain in most cases numerous elements of the story. Its most likely, than when directors or producers or whoever decides, "Lets make a movie about a game" will focus on whats in the cutscenes first and formost, cause it likely is the easiest way to get the story.
2. Executive meddeling.
Pretty simple. Whenever Hollywood gets a videogame franchise, they tend to alter said franchise. While some games need to have some story elements changed, alot of changes can also be argued as more in line with makign the movie how the directer wants it. Just look at Doom, the games series basically had the forces of Hell attack space marines on mars. The movie, instead of the forces of Hell, its a gentic anomoly that causes mutations in most people,save (obviously) the one main character who didnt die and wasnt a d-bag (sorry Dwayne).
3. Fan expectations.
This isnt Hollywoods fault, this is our fault. The most likely target auidince four videogame movies are the fans of said videogame. So you can expect the obvious level of expectation for a film from series fans.