Actually some people wear prescription sunglasses. It's not always easy to switch back to regular glasses. Sometimes you leave the plane with sunglasses, and only realise in the terminal you are still wearing them. Your regular glasses are back in the plane. Do you quickly go to the washroom with your sunglasses, or run back to the plane through the crowded jet bridge to get them?
Yep. I'm one of those "some people" too.
It is very easy to switch back to regular glasses. You keep one case or both cases on top of everything in your flight bag. Then, once the airplane is parked and you think you need to run into the terminal, you swap out your glasses. Hell, tie it to taking off your headset.
I don't know about you, but the need to take a leak doesn't just come on. There is normally a little warning. Enough warning that you can think to yourself "hey, I need to swap out my glasses when I go into the terminal."
It truly is the little things that differentiate pro-ams from professionals.