ppdouble: I dont (nderstand why soundtrack should be separated in first place. Like it is part of game . Why separate it? It should be free and in mp3 format by default.for evry game...
Because the game and soundtrack are likely made by separate entities, so already separate.
It's not just about GOG doing a deal with the game provider, it is also often about the game provider doing a deal with the music creator.
And music or audio creators are separate entities in most cases, and not really part of the game development team, not in the same way that other members are anyway. That likely means the soundtrack has a separate license, a license that often expires once a game has stopped being published. That would then require a new negotiation in any subsequent re-release ... unless that has already been covered in the original deal.
Part of the deal usually, is the music creator retains the rights and can sell the soundtrack independently, and they often do at Bandcamp and SoundCloud and other places.