Because it fucking HURTS to get hit in the face when you have glasses. Especially when you got the nosepieces.
I've had a basketball hit me in the face, I've beaned someone in the face at a volleyball tournament, and I've kneed myself while doing a roll off a gymnastic horse.
Besides glasses still being expensive, my parents couldn't afford to get me new glasses if I broke them. I can't afford to get them now. I rather not have to deal with what my older brother went through when he got hit in the face, and his glasses shattered, and his face was a bloody mess.
So fucking don't hit me in the fucking face. And unless you want to be a complete douchebag who's known for hitting people with glasses, don't do it. Or I'm going to send you a bill for getting my eyes checked/medical bill if I need stitches, but I'm going to go on all-rage mode, and I may be vision-impaired, but I'm not that blind so I can find you, and I don't care where my fingernails go, but I will make sure that you hurt. And that you hurt bad for basically blinding me.