Why don't QBs wear Visors?

Me and my roommate were thinking about this one.

You know how the analysts always say, "He was watching the QBs eyes, and jumped the route perfectly!"?

I was thinking, wouldn't wearing a Dark Visor prevent DBs from watching a QBs eyes?

I imagine it would be a little harder to "look-off" a Safety..... but hey.

* I also remember Colt Brennan wore a Visor a lot too, I wonder if that helped him at all.

** My apologies for the stupid question. I imagine the answer is pretty simple and basic. But I just had to ask. :D