On GOG for instance a OST is included with many games for free.
Actually it's not though it's easy to see why you'd think it is since it's in the same spot with the rest of your goodies. In reality it's part of the Source Hunter DLC, look at this page, not on your own game shelf to see what people who only buy the game get:
http://www.gog.com/game/divinity_original_sinThe artbook, design notes and soundtrack are part of the Source Hunter DLC, and don't go with a regular copy of the game.