I don't know how it works, just guessing. If the specific copy of the game was sold through Steam, then Valve is making money on it, and they are expected to provide the achievements (which they do). If the game is NOT sold through Steam, then I don't expect they're making any money on it, so they may require the developer/publisher to pay for the service, and CDPR chose not to, as TANSTAAFL. Or maybe a lot of the contracts say that Steam have to do it for all copies of the game, rather than just those sold through Steam, but CDPR didn't include that clause. Or maybe it requires the use of certain APIs in the game and CDPR didn't include them in the non-Steam version.
As I don't know the reason, I was wondering if anyone here did. I'm struggling to think of a way that would make it Steam's responsibility and not CDPR's, so maybe you could clarify on your reason for thinking that? I'm not looking for an argument on it, just curious about how it works.