ThermioN: Now, you have to buy it separately for no reason than to milk the cow some more.
In the old days (take it how you will), when someone made a game, they bought the rights to the soundtrack as well. So all the assets of the game belonged to the publisher, and they could sell it however they liked. Once people started being able to sell music through the internet, some music composers made deals that allowed the publisher to pay less for the game music, but the composer retained the rights to said music and could sell the music themselves. So while the publisher could use the music for the game, they couldn't sell the soundtrack themselves.
For the publishers, this means the lower the cost of their games (and by not lowering the game's price, increase their profits as well). For the composers, that means that they (may) get more income, since they can sell their music twice (and repeatedly). For the OST lovers, it also means that they can get the music without buying the game.
The downside is for the collectors who want to buy everything in one package.