I don't think it's about courtesy, necessarily, more about that many just don't realize they're piloting a two ton death machine that can kill people in the twinkling of an eye if they don't pay attention. It's not discourtesy, it's just stupidity and carelessness - like when one idiot stops in the left-hand lane to chivvy some ducks off the road, killing two riders who plow into the stationary vehicle, or another idiot is literally applying makeup while driving to work, and many other incidents where drivers just don't realize that driving a car is something that requires 100% of your attention at all times.