Why don't QBs wear Visors?

Ya'll are correct about the doctors orders to wear the dark visor, the player has to have a condition where the bright lights affect them.

I would imagine that visors could also decrease visability by getting dirty and scratched up. Especially in open stadiums with grass and dirt flying around. At a position where clear vision and split second decisions are needed at all times any slight smudge or scratch could hurt a QB's accuracy and timing.