I can answer a little. It's a problem of the conjunction of providing enough resolution that the screen is useful while having a large enough viewing angle. If I want to replicate the screen for my Surface Book, 3kx2k, at normal working distance that screen takes up about 1/5th of my working vision (that being the area my glasses cover which is a majority of my whole field of view and as much as I could expect of a non-contacts AR solution). So to render that screen and be able to place it inside the normal working field of view any AR glasses would need to have a resolution of about 5-6x whatever screen you want to duplicate, and this number gets worse as we talk about replicating TV screens which are further away and thus need even higher resolution to look the same.

It's a problem of being able to pack that many pixels onto the tiny screen area of a glasses based AR solution and then being able to process and render that in a mobile form factor. It's a hard problem to solve to say the least.