Shit, I assumed that valve somehow got a cut of games from keys as well, but looking it up (briefly), it looks like you're entirely right and they don't.
That makes it even more bonkers that companies keep trying to siphon off the market share, since you could just take your market proceeds as bonus revenue as long as valve got their share of what they sell.
I'm assuming that's a big chunk of how things like humble bundle make their money?