Developers go where the money is, and that is Windows. To support Linux, they don't need to do anything. To support Steam Deck, they usually don't need to do anything, except maybe adjust text size based on screen resolution. That's it. To support macOS, they need to support a completely different rendering pipeline, ensure all of their dependencies support 64-bit, and now potentially target ARM (Rosetta, the x86 -> ARM layer can have a big performance penalty). Also, Macs don't have very powerful GPUs, so even if they do support macOS, the bigger titles will probably run poorly anyway.
So what are they going to do, put a ton of dev resources to properly support macOS, or put one guy for a couple weeks to support Steam Deck/Linux?
It has absolutely nothing to do with cost of hardware and everything to do with cost of supporting the platform vs number of potential customers.
A couple months ago Apple released some code for porting games that supposedly handles having to write custom code specifically for Metal. It's based on Wine so it seems similar to Proton, but Apple probably wants developers to package their games and get them approved for sale on the app store or whatever instead of having users just running their own games. It looked hastily put together when I checked it out, but I don't develop for Macs anymore so I haven't actually used it. It's too soon to tell if game companies care to port their games and go through retesting that the graphics work using the API translation layers to run on different drivers and hardware.
I asked someone who is more familiar with Apple. The thing is you can use it to play. But, the license is for testing games or something similar. I don't remember the exact wording but it's not intended for the end user, but devs. So, Apple might crack down on some people using the porting tool kit to play games. They might not, but it remains a possibility.
I don't get the "Game Porting Toolkit" they made, content-wise it basically looks like a regular Wine packaging - much like what Proton is, but then it has one of the strangest licenses I've ever seen for something designed to help development and shipping.
To paraphrase, you can't include any part of the toolkit with your product. Not the development components, the runtime components, the translation layers, nothing. So good luck using it to actually ship game ports, since that would be a license violation.