Physics. A quest is basically last year's mobile phone pushing way more pixels and FPS than on a phone. Because VR is heavy. And needs low latency so some optimisation tricks are out the window.

Even PCVR with my 4090 is still very limited in quality in things like Flight Simulator.

Meta could have gone the Apple route but they price themselves out of a real market. And the external battery is really poor UX. And really it's not super much better (though they compensate a lot with foveated rendering)

This is basically what the tech is right now if you really want standalone.