The key question here is the ergonomics. It takes time to adjust you focus near-far and you can't just stick a GPS on your shield and make it work. It is distracting and dangerous. Cockpit HUDs have different spatial relationships with your eyes.
HUDs use light focused at infinity so the point of focus for your eyes doesn't change when looking at the display (it "hovers" mid-air). Holographic sites, HUDs, Google Glass all use the same principle, with varying ways of getting the light onto your eye (curved mirror that changes the focal point of the light properly that reflects on a piece of glass, a piece of glass curved to focus the light properly when it's projected on it and a lens on the projector to focus the light properly are the 3 primary approaches).
Wait until you hit 40. I wonder if my old eyes could even focus the HUD at all.