Why Do People Drive Stupid?

Is it genetic? Is it environmental?  Or is it just plain old-fashioned stupidity?
Why would you drive in the dark AND the rain without your headlights on?  On my way home, the road was filled with vehicles that did not have their lights on at all, while some made a feeble effort with just their parking lights.  To top it off, I seem to recall that most states have laws that require the use of your headlights when your windshield wipers are operated.  Believe me, if these cars weren't using their wipers, they either have a death wish or must have serious Rain X.  So lawbreaking idiots without your headlights STAY OFF the roads when normal sane people are driving.
And for another thing, it is not illegal or immoral to use your turn signal.  It is more than appropriate to turn your signal on when merging, changing lanes, or dare I say, turning.  I can not fathom why it is so difficult for people to extend their little finger a mere two to three inches in order to activate their turn signals.  Even if you call them blinkers, let them blink!  Use them to share your intentions with the rest of the world.  Save yourself and us, a lot of stress from the parking lot to the interstate.  As Martha Stewart says, blinkers are a "Good Thing!"
Update 12/1/2010
Why can people not put down their makeup, their newspapers, their cell phones, their breakfast and whatever else is distracting them, and DRIVE?  On the way to a seminar yesterday, I observed at least ten people that didn't even have a hand on the wheel.  Grrr!  I know that some people are blessed with talented knees, but really? 
And then there were the HOV lane hogs who were sailing by, all alone!  I understand that the state would lose money if they eliminated the carpool lanes, but how about a little bit of enforcement.  I lost count of how many violators I encountered on my way to and from my meeting.  Do people really feel that entitled to special treatment?