Why do people feel the need to drive four inches away from you when they know you can't go any faster or get out of their way?

So living out here in California for the short amount of time I have, I've noticed a couple of things, like the weather is awesome and the traffic sucks really bad. But recently, paying a little more attention to the latter, I have noticed that not only does traffic suck, but people in California don't know how to drive for sh**.  Every day coming to and from work, a good 45 minute commute, I see an accident.  Granted there is usually a lot of heavy traffic, but c'mon, an accident every day in my 25 mile commute.  Not to mention that I have been victim of these accidents, I have been hit from behind twice in the two years that I lived here and both times the person that hit me ran.  

It's a given that anyone who lives out here knows how many accidents are caused by tailgating and yet everyday on my to and from work it's inevitable that I'll get some jerk, usually in a lifted 4x4, riding my *** so close I can't see his license plate.  Granted I am not the most astute driver, but compared to most the drivers I have seen out here, well, call me Mario Andretti, whoever that is.

As I was looking for some car crash pictures I came across some ridiculous ones, heres a few more. I wish I still had the ones from my hit and run's