I think it's because we haven't thought of enough interesting things to do with them. Sci-fi HUD displays are mostly cool because of the applications, not just

I think it's because we haven't thought of enough interesting things to do with them. Sci-fi HUD displays are mostly cool because of the applications, not just their existence. If you only use something a couple times a day, it needs to be absolutely seamless or you're going to take it off and forget about it.