This was (going from memory here) due both to the much higher refresh rate that's required, to having to pre-render not only what the user sees but the whole 360 degree sphere they might look to around them at any moment, and perhaps some other VR-specific requirements that don't spring to mind at the moment.
Was I dreaming that I read this? Maybe a VR developer or someone more knowledgeable than I could comment?
The parent post is probably referring to foveated rendering - a research technique which uses eye-tracking to render most of the screen in low quality and areas being looked at in higher quality. (Real vision is perceived something like this - try reading text that isnt in the center of your gaze.) You can do foveated rendering better for a close screen so eventually it's possible that the quality of VR rendering will surpass regular rendering.
[1] docs.unity3d.com/Manual/SinglePassStereoRendering.html
Even better, if you render just a couple degrees extra you can double your framerate via interpolation. That way you can have 120Hz head-tracking while only having to render the world at 60Hz.
Future headsets can use internal cameras to track the position of the pupils to figure out where that sweet spot needs to be every frame.