I'm no game dev, what I gather is that Apple hasn't been very friendly in terms of graphics drivers and compatibility in general. There was that article a little while ago where some company implemented a fully compliant Vulkan for Apple Silicon better than Apple did? If Apple makes it more difficult almost on purpose, why should developers come to the table when their target audience isn't on the platform?

I get the impression that mac-specific tools work very smoothly and nicely, but cross-compatible tools suffer needlessly.

For Valve at least the decision seems to make sense - Linux has overtaken macOS in their hardware survey. No doubt Steam Deck and Proton have a hand in it.

https://www.gamingonlinux.com/2023/10/september-2023-steam-s...

And it's just the start I believe. Nobody want to admit it but Apple is extremely uncompetitive with their current hardware offering that is completely locked up. Those companies sees that it is not worth investing in this dead end. They understand the clients they have will figure this out eventually and switch with them in the long run. macOS doesn't offer anything special that would warrant focus in itself and the hardware can be interesting only if you want to lower power consumption for some reason (without considering other factors).