That's mostly because most people don't think about trucks at all. And economically speaking you're definitely right. But that isn't to say that there arent other ways that self driving cars can be transformative. I mostly walk everywhere, and at least 2-3 times a week I run across a hurried or inattentive driver that can't be bothered to stop before turning right on red, or turning left after waiting for traffic to clear without looking for pedestrians, or stop for pedestrians at an uncontrolled intersection. It would be a huge benefit to my quality of life to not have to worry about being killed by a selfish asshole like that.