Here's a little experiment:
The next time you watch a movie, or re-watch your favorite movie, imagine playing it verbatim as a video-game - every single bit of the movie is part of the game.
Think of how un-fun it would be. Of how jumbled the pace and game-play would be. Where's the boss fights, how long do you have to sit through dialogue scenes, or cut scenes, when or if do you actually get to explore the open-world of the game (movie), and how long is it from the start of the movie till the first bit you get to shoot a baddie?
Imagine 'Children of Men' (there's the stealth mission where you have to sneak out of the farm), or Pulp Fiction (as a point and click adventure game?)
This is basically the reverse of what happens when some poor sap is tasked with adapting a video-game into a movie, or a book into a movie, (or if they're a really really really poor sap, a movie into a book).
It's not that it can't be done right, it's just very difficult because video-games and movies are completely different forms of entertainment and of story telling.
Video-games are non-linear, you control the character(s), expected to be longer than a few hours. Movies have a beginning and play until the end, follow the character around, and do it usually in less that 240 mins.
Further more there are things you can do in books that don't work well in movies, and vice versa, so people who would adapt say, Half-Life 2, into a movie has to understand that it just wouldn't f***ing work. No one wants to watch a mute bearded man drive along a coastline, occasionally stopping to punt boxes into the ocean, for half a movie.
(wasn't sure if he was aloud to swear on this website)
