Because there are screens in the way? The choice was to either not have the viewer's eyes be visible, or use a screen to display eyes (not even real eyes, you can supposedly have cat eyes for an example). Considering the device is meant to be AR (augmented reality) and not VR, it kinda makes sense to show the user's eyes since they're still "connected" to the outside world. Otherwise you'd have a bunch of blank visors walking around and then people can't tell if you're looking at them or your furry waifu.