Apple built that already, the Game Porting Toolkit - it's just not as seamless as Proton, because Apple doesn't have all the windows binaries on their servers to give to users.The magic of the Steam Deck is Proton, which allows you to play the vast majority of Windows games pretty seamlessly without Windows. Without Proton, Valve's investment in Proton, and the deep integration of Proton in SteamOS, the Deck would be just another handheld PC. There were dozens of them that were completely forgettable before the Deck, and there will be dozens of after. Proton is what makes it special.
The core problem is the interface and market. For Mac, the market for a AAA game sold through the app store is tiny, because users have been conditioned to not look for games there. And if you go for iPad/iPhone (A much larger market) you have to invest into making control schemes that work with touch, because you can't sell an iOS game that requires a connected controller. And you have to somehow balance the low end 32GB iPhones and high end ones in terms of asset storage (Which the app store doesn't help with, but Steam does.) And then the biggest problem - your $60 AAA game is competing with 10k freemium games that have 1000x the number of downloads/ratings/engagement which means you get lost in the app store search results.
Hence, no ports. The tech is there and ready, but Apple corporate is not ready to budge on software going outside the app store, or changing the app store to support anything other than microtransactions & subscriptions.