Me too. I move to Kansas from the east coast about 10 years ago, and started to see this all the time I-35. It really makes me mad. Anal retentive control freaks.
I don't think it's that. Modern vehicles are very efficient at reducing outside noises, and it's fairly easy to slip into a semi-hypnotic cocoon while driving. These people get beside someone and pace him or her because their brains are a million miles away and they figure they can't be speeding if they're going the same speed as the guy beside them.
In other words, they go brain dead.