If a developer is not willing to lift a finger to port to mac (a small market, but one with a known size), why would they port to Stadia or some other unknown market?
But more importantly: Mac hardware usually isn't really equipped for high-end games. If you have a pro-tier machine you might do okay, but nobody buys Macs for gaming, at the very least. It's just too niche of a market to go through a lot of effort to support it
The latter isn't a niche market, it's a 'not high-end' market. But that could evolve, I think.
Whereas GNU/Linux, even with the massive amount of games targeting Android, hardly gets to see them.
Same applies to Stadia, which is mostly GNU/Linux + Vulkan, with Google sponsoring Unity and Unreal as well.
You'd think, but a lot of mainstream engine-based games that could "easily" have a mac port never get one, even an unofficial one offered as totally unsupported. Look at Among Us for example. Not by any stretch a high-end game. It runs on Windows, Android, iOS, a bunch of XBoxen, and probably other consoles. I bet the developer could spit out a working native macOS version with the push of a button, but so far hasn't.
Kerbal Space Program is another example. When last I checked, they did have a native mac version, but it was hamstrung in some way--I think it was limited to 32-bit or something.
I can't imagine these examples are actually a huge amount of effort to make happen. As a fan and programmer I'd be willing to do it for free.
A lot of games did drop off the Mac when it moved to 64-bit only though.
I imagine when Apple expands their desktop and laptop lineup to M1 chips, it's going to include many of the games that are available from their mobile catalog.