Besides, the main reason Valve is investing so heavily in Linux and Proton is so their destiny isn't tied to someone else's platform. MacOS is just another someone else's platform like Windows is, with the same threat of getting rug-pulled by a first-party app store that spooked Gabe Newell[1] into investing in Linux in the first place.
The magic of Proton from a consumer point of view is that it just works for basically every game, sans those with Kernel-level anticheat stuff. This means thousands of old games that haven't been updated in years will work.any games that don't have active developers.
So Apples solution works for new games but isn't a practical option for compatibility for existing games.