Apple and YT would miss-out on advertising to a captive, targeted demo of UHNWIs, corporate types, and those with expensive toys. Second to being a UBS or Tesla customer list or list of people with ASW accounts, it's a target-rich environment of that particular demo. Of course, marketing to that demo is very precarious and must be done deftly because it could easily backfire.

these people already bought YouTube premium, there's no advertising from YouTube

I highly doubt VR is going to replace a phone for users. People will still sit in bed and stare at their phone. vr is more of a hobby than it is an everyday computing device

I think the fact that apps are available for Meta's products show that (at the time) there was confidence into a potential growth category. And (currently) there is no confidence that Apple's product makes any commercial sense (in the near future). This is even emphasized by Apple clearly launching this in small quantities with excessive margins.

Meta funded some of those apps.

Meta developed the Netflix one in-house for example.

I doubt Apple would offer that.

The opportunity is to launch on the Vision Pro developer/early adopter device and get your app ready for the mass market version.