I think it's staggeringly incorrect to say games don't have good music. Most AAA games have music on par with films these days.
- On the one hand, there's games with licensed soundtracks. Tony Hawk, Need For Speed, Burnout Paradise, that malarkey.
- Then there are games which have music integral to the experience. Jet Set Radio, Rez, Space Channel 5, Dance Dance Revolution... shit, I just named my favourite Dreamcast games, didn't I?
- Some games however just wouldn't feel right if they were accompanied by the constant thump of RnB. In some games, ambient sounds, incidental music or plain old silence just work better. Just because you're not bopping along to some sweet tunes every time you have a blast on Gritty Urban Shooter 4, doesn't mean the developer is "doing it wrong".
- In the case of incidental music, sometimes the hallmark of success is discreetness. The music shoud be complementing the action but playing second fiddle (quite literally).
- And finally, I suppose a reason why a lot of modern games don't have "soundtracks" as such but rely more on background music, is the need for the protagonists to be heard talking. Going from Monkey Island 2 (text dialogue) to Monkey Island 3 (spoken dialogue) it's quite noticeable how the volume - and quality - of music dips in the later game.