True, but if you think a lot of words or phrases that have become common usage, they don't make a lot of sense... or they make sense in ways that can be disturbing (i.e. my grandmother always used to use "cotton picking" as her generic curse word equivalent, and I never thought about what that might have meant until I was 22 and it occurred to me for the first time who was doing the cotton picking).
Chromolume has pretty much summed up my response... I used to correct everyone when they said soundtrack, it was basically a Pavlovian response to immediately say "Cast Recording", bur, you know, it just seemed like more and more of a niche awareness and that Soundtrack was becoming our generic phrase for "songs from entertainment". And at the least, it's efficient. Two syllables, rolls off the tongue easily.