Why does VR need so much powerful hardware if the graphics are

>>59322223
VR games have to respond to much more nuanced changes in the angle you're looking at than most games, where panning at a speed given by a mouse or joystick is much more simple to implement.

I actually know nothing about VR implementations, that's just my best guess.

You need to understand a bit how the computer graphics stack works.
The most resource intensive part is rendering stereo 3D from two different perspectives (each eye @ 1080x1200) at the same time. An inefficient implementation of this would require double the GPU/CPU power, but thankfully there are some optimizations in place. Not enough though.
On top of that, VR runs at 90fps as opposed to 60, and is extremely sensitive to frame drops below that target.
There's some extra overhead for rotational+positional tracking of the headset + 2 controllers but that's primarily CPU overhead. Will also impact performance a bit.
Lastly, extra shaders required to correct lens distortion are also in place and take up some processing I believe.

Combine all those and the minimum spec of a GTX 970 seems very modest.

>>59323428
Also have to render each scene twice at different angles @ 90fps minimum.