Okay, let's think of the practicality of having mirrors on your glasses or your bike for that matter.

If you're sprinting to the line at 30+ mph, do you know how steady you'd have to hold your head or your bike to be able to get a clear picture of anything behind you.? I don't know about you, but when I'm out of the saddle sprinting, or even giving max effort while seated, the last thing I'm going to want to do is try to hold my head perfectly still and at exactly the right angle to see what's behind me. There's just no way in the "heat of battle" having a small mirror mounted anywhere on you is going to make any sense. You're going to be rockin' and rollin' and so is that tiny mirror.