This is why.  A portrait with glasses, in which we shot into an umbrella. Looks fine, until you zoom all the way in, when you see this:

See the light?

Now of course the next shot is “tilt your head down” shot so the glasses no longer reflect – but the eyeball will still reflect a white circle with a black light inside it. Can you see it, in both eyes?

Shooting through the umbrella instead would have shown a nice white circle, instead.