I see people answering as if Steam/Valve is the one looking for game, for some reason I'm pretty sure the developers go to Steam to sell their games and not the other way around.
And game developers really have no reason to add extra's to the digital download, they just want to sell their game via as many venues as possible.
GOG on the other hand does contact publishers/developers to negotiate releasing the game via their site. They need the extra's to pull in the customers, who often are people buying the games out of nostalgia. And more reasons of course, but basically I think it boils down to the fact that GOG has to negotiate with the publishers and the publishers negotiate with Steam/Valve, different business models.
That's my two cents anyway.