Because their gaming market would be tiny. Gaming on Mac is completely dead, and mobile gaming will take at least a lot of time to adapt to the hardware.
"Dead" is an unnecessary exaggeration. I'm a Mac gamer and my Steam library has more Mac games in it than I can possibly play all the way through. No Man's Sky was just released for Mac, and I'm looking forward to playing that too. I just played through Subnautica at the same time as my friend who was playing it on his Switch and he was blown away at how much better and smoother it was on my M1 MacBook. Also Parallels and Crossover open up the ability to play a lot of Windows games on a Mac. I'm still impressed with just how well that works for some games. I'm not a bleeding-edge everything-in-my-life-is-about-gaming gamer, sure, but I still think I'm a gamer. Yes, compared to the Windows gaming market, the Mac gaming market is small, but it's not dead.
Paid emulation, but not as slick of Steam's Proton (which one could get for no extra cost). Linux currently has better gaming support than Mac.
Gaming on Mac is not "completely dead", but it's basically getting leftovers. That's ok for many people, but it will negatively affect the fortunes of "Vision Pro".
No, but imagine getting DOOM in 1999 instead of 1993.
As I was saying, Mac gaming is basically leftovers. Which isn't terrible, but means the Vision Pro probably isn't very good for games (raw performance is fine, but devs won't build for it).
Some other things mentioned at WWDC besides Vision Pro: The new macOS is getting a "game mode" for better performance during gaming (even though it's already pretty good!) and they're making porting to mac easier too. Oh, and Hideo Kojima was one of the presenters and talked about bringing his stuff to mac. Apple is clearly starting to court gamedevs and gamers, and I'd be willing to bet that what we've seen so far is just a start. Apple doesn't usually half-ass things.
But it's not a mac and gaming is the current biggest application for VR. Apple could have gone on a spending spree and built up a family of games if they were interested.
