Games run primarily on music loops. Each piece is composed to cycle during gameplay so it's important to create music that doesn't get monotonous or annoying and has very exact pacing. The use of themes, signature motifs and rhythmic passages needs to be arranged so they remain effective when played over and over. In a film, it's a one-time-through experience. Tracks are scored tightly to picture and each track is crafted to exactly match what's going on on-screen. Scoring in such a precise manner wouldn't be practical in games because of the dynamic environment of gameplay. It can be different each time you revisit a level or area of a game, whereas a film is intended to play one way every time.