Why is it that the worst drivers can be found on the interstate?

It seems everyday on my commute to work I come into contact with some of the most ignorant people who are severly vehicularly challenged. It's like they have no idea though they drive the same path everyday which lane they need to be in and wait until the last minute to get over for their exit even if it means cutting another car off. It happens mostly within the city. Also I would almost swear that if you dumped a gallon of water on the road that some idiot will wreck because of it. While I am at it why do people consider the left hand lane of the interstate the fast lane when inevitable there is some old man or woman in that lane who goes 5 miles below the posted speed. I mean really if you are going to drive slow get the hell out of my way, I have somewhere to be obviously you do not. Has anyone else noticed this? Surely there are others out there like me that wonder how the hell some people managed to get a license in the first place.