I actually see it as the reverse. Valve might be going for the whole pie and want to carve out a niche for their Steam Box. Inviting Apple to the party might detract from that effort. Or at the very least distract from their main focus.
I don't think Apple wants any non-Apple store addressing their weaknesses, especially a solution as competent and well-funded as Steam.
If Valve gains Apple-user mindshare on Mac, what prevents them from expanding to iPhones and iPads in the EU, and likely elsewhere if anti-monopoly laws get entrenched? IIRC, Services is the fastest growing revenue source at Apple.
They don't need Apple for that. People who game already game elsewhere. Steam on Apple feels pointless. I wouldn't be surprised, if Valve will go for smartphones with their own at some point
A phone that can run my Steam library is super-compelling -- I travel a decent amount, so being able to chuck something smaller like a Backbone One in my bag vs. a Steam Deck would be a meaningful change.