Because the learning effect is zero. (Also, not every professor allows it, and you don't always sit in a location from which you can take a decent picture. But that's beside the point.)
The best teacher I ever had in school never got tired of reminding us that we learn in four different ways: hearing, writing, speaking, drawing. Taken individually, hearing is the worst, drawing the best. The more you combine, the better your rate of recall will be.
Drawing is fantastic because it engages your kinesthetic learning, your spatial learning, and your analytical learning (because you really have to look at a picture in detail before you can copy it).
