The point is that apple is expecting a chicken to hatch out of an egg that doesn’t exist.
Devs will use Metal and apples other proprietary systems on mobile, because a gaming market that has passed critical mass already exists there.
Apple is expecting devs to be willing to do the same on desktop and laptop, before the gaming market for those platforms have hit critical mass. Yes, there are “some” games, but right now, apple simply isn’t doing what’s necessary for it to take off for real. Devs, gamers, and Valve, know that.
Valve was willing to put in the resources back when apple was doing the same, but they stopped long ago and have only been doing the bare minimum of lip service, since. So, Valve packed up, and became re-focused on linux instead.