To understand why, consider how we see.
Our vision consists of a high resolution fovea in the centre, surrounded by a much more blurry periphery, which has evolved to be good at detecting motion but far worse at fine detail and colour.
Graphics card maker Nvidia has calculated that around 96 per cent of the pixels in a VR headset are viewed in our periphery, rather than the central fovea.
The technological race is therefore on to develop "foveated rendering": headsets that display a small, high-resolution spot that follows where we are looking, but a steadily lower quality image in our periphery, in order to save on processing power.
But if our entire visual field had the same acuity as our fovea, we'd need an optic nerve and brain perhaps a hundred times bigger to process all the information.
Nvidia shed some light on why: our ability to detect the presence of contrast – the difference between light and dark objects – in our peripheral vision peters out more slowly than our ability to resolve details.