The CPU/GPU part of the device does not need to go on your head. Manufacturers make it that way for convenience (and lower part count) but that doesn't necessitate that stand-alone headsets be made like that. Apple, for example has the battery in a separate little dongle that you're supposed to put in your pocket. If they can do that there's no reason they couldn't put the CPU/GPU hardware elsewhere too.
The Steam Deck can play some VR games just fine. It just wasn't made for that purpose so as it stands it's currently sub-optimal. With a few minor changes that very same chip/hardware could make a decent VR headset.
So you want powerful enough PC on your head? It's impossible for everyone. Valve doesn't make a chip. Even for Apple, their chip is great but not comparable with powerful GeForce.