They have done that.
They gave some cash money to CodeWeavers, the company that created wine. It's called the Game Porting Toolkit: https://developer.apple.com/games/game-porting-toolkit/
As far as I know, CodeWeavers was not involved in that project:
> We did not work with Apple on this tool
https://www.codeweavers.com/blog/mjohnson/2023/6/6/wine-come...
They have not done that, though. With Proton, almost every Windows game on Steam "Just Works" (and many others do with a small amount of configuration.)
As far as I can tell, there is no way for a player to use GPT to run games from their Steam library on Mac.
Proton is so good that devs have dropped Linux builds for their games because Proton runs the Windows version faster - emulated.
That's the problem I think: porting.
As I understand things, proton allows windows games to just work (pun intended) on Linux. No porting, no rebuild - just download and run.
Who is going to bother doing all the extra work to port their game for Mac?! Time and time again there have been loads of articles on here over the years with developers saying it is simply not worth the hassle to support Linux and Mac.