Obviously their motive is to sell games, but their priorities likely don't currently include Apple's hardware as a result of wanting to compete in the console market and wanting to twist away Microsoft's near-monopoly on PC. Not everyone is a Linux user, but there are lots of people who would be willing to use it if it was sufficiently packaged. The Steam Deck is likely to be exactly that.
Fine, they want to sell games. They can still sell games to Mac users. If the back catalog isn't quite there, that's probably not a huge deal, as it's not exactly where the most money is made.
Could they go ahead and license Crossover, today, for every Steam user that tries to open a 32-bit game on macOS? Probably! Could they slap together a distribution of Proton for Mac? Absolutely (and for a time, it existed)!
But my guess is that the Apple juice just isn't worth the squeeze. It's probably not super important to their strategy, it's probably not going to yield back enough profit to be worth it, and if it were really so easy I suspect they would have furthered their work on Proton for Mac (https://github.com/ValveSoftware/Proton/issues/1344).