Couldn't Apple spend their resources on that? Proton is open-source, and Apple's the one with the incentive to have more "prestige" AAA game devs to parade around during keynotes.

Apple could but they're not interested in non-native games, they want native ports or nothing. As I discussed a few posts over, Apple went to the trouble of developing a DirectX compatibility layer, but then told game developers they're not allowed to use it for anything besides evaluating whether their game would run well enough on Mac hardware. If they go ahead with a port then Apple still expects them to do it all the hard way.

It's textbook "perfect is the enemy of good" because yeah, compatibility layers have overhead, native is better, but if you insist on native everything but can't get devs on board then you just end up with no games.

Nope because they could not gouge developes with pricy tools, steep registration fees and cutthroat slice of their sales on Apple's app market.

Apple could but Apple would rather die they allow something to work cross platform.