Hair is very difficult to draw, because what we see is different than what we know. You can’t simply draw all the hairs in the form of lines, because that’s not what we see when we look at hair.
The pattern our eyes take when we’re drawing something is sequential. They don’t flick about nearly as much. … But instead of our eyes flicking all around the object we’re drawing, a typical zigzag pattern, they creep around the outline a bit at a time, pausing as they go. That’s what makes drawing accurately so hard.