That's a fair point. It would be easy to categorize them in their own sub-genre. Some soundtracks are just collections of previously recorded songs from pre-existing albums, like Dazed and Confused, or Guardians of the Galaxy. Still, they're great collections on their own.
Miami Vice, on the other hand, had a great collection of songs from various contemporary artists capturing the feel and the lifestyle perpetuated by the popular series at the time. Less Than Zero, however, had an interesting mix of rock, rap, r&b, and metal covers. Perhaps the most notable track was Slayer doing an abridged cover of Iron Butterfly's In-A-Gadda-Da-Vida. But the most iconic track, IMO, was the surprising hit from the Bangles, A Hazy Shade of Winter.
But that leads into the category of original scores. You might not think of this as regular music the majority of us would listen to casually on a regular basis. But you can't deny the power and emotion of some of the best known and iconic scores, especially when they're tied into a beloved movie or scene. George Lucas has said that the music was the only thing that exceeded his expectations for the original Star Wars movie.
Conan the Barbarian was another standout of original scores that helped define the movie picture. When I hear the opening drums and horns of The Anvil of Crom, I am instantly transported to the days of high adventure. And let me tell you, there is no better way to wake up feeling ready to take on the world than setting your morning alarm to this. Try it. (Just make sure the volume isn't so loud you wake up with heart palpitations! I am speaking from personal experience on that one.)
Edit: I need to throw in one more--Last Action Hero. I love this movie, and the soundtrack is one of my favorites ever. Most were original songs that were hits in their own right (like Big Guns by AC/DC, Angry Again by Megadeth). But it also featured two new songs from Alice In Chains (What the Hell Have I? and A Little Bitter), which I think was among their best songs ever. And if that ain't enough, let's throw in some killer tracks by Anthrax (Poison My Eyes), Tesla (Last Action Hero), and Queensryche (The Real World).