Well, first, because they're just learning. They're over confidant and daring, and they think they know what they're doing when they don't. Or, overly cautious and jerky, nervous in their actions. Also, they try to impress people. Whoever they're driving with, friends or family or instructor, is there to be impressed by how well they handle the car. And they expect too much out of themselves. No one wants to be the statistic, and the whole world is pushing them to drive right. Learning to drive sucks.

I should know. I'm on a learner's permit. :P