I can think of many reasons why it would never happen but it sure would be nice. Not that I haven't been voting against Mac games with my wallet for years, I've had a Mac to get shit done with and a rotating set of consoles to play games on since about 2000, and very occasionally bought a point-and-click adventure for the Mac.
* Apple has billions of dollars and a hierarchy of decision-makers who could prioritize the R&D and implementation of making "gaming on Mac" a reality
* Linux is a distributed, community project without billions of dollars or top-down decision-makers who can unilaterally prioritize making "gaming on Linux" a reality
I'd say Valve is exactly that. Valve pays many developers their salary who are responsible for making almost all single player games work on Linux via proton (wine, dxvk, vkd3d). Although they do push their changes upstream.
No it's not. Valve is just a member of the community. It cannot for distributions to adopt anything. Valve cannot prevent kernel developers from making it difficult for them to improve gaming on Linux. Valve just does its own thing and offers up its work. They cannot force anything into Linux like Apple can with its OS.
Perhaps unsurprisingly, the Steam Runtime is considered a bit of a mess on the technical end; Valve used to only test it with Ubuntu LTS, which caused plenty of issues for those on other distros or on newer versions of even Ubuntu.
IIRC Arch literally comes with a package whose sole job is to substitute certain steam runtime libraries that are known to cause conflict problems when used, otherwise you get fun X Window errors (or whatever equivalent you have for those on Wayland).
1. Apple has multiple teams of well paid engineers to solve any problem that arises. There's a lot more money in the game supporting Mac than Linux.
2. Apple has been shown to be hostile towards game development and make active decisions to make it harder to port to them, whereas Linix's most hostile issues arise from the proprietary nature of games vs. the open source nature of Linux (e.g. Package management, DRM, etc).
3. Linux makes concessions on its philosophies for games while Apple makes ultimatums. You can't ever trust that your game on Apple will work in 5 years, at least if you were developing in the 2010's. Meanwhile there are proprietary ways to deliver your game if you want to launch on Linux (not as sure about DRM but I've heard of solutions that simply haven't had mass adoption yet).