Because often someone is hogging the left lane but not the right lane, especially in San Jose where nobody wants to be in the right lane due to the wacky merges

Because often someone is hogging the left lane but not the right lane, especially in San Jose where nobody wants to be in the right lane due to the wacky merges. So it becomes the true passing lane, with extra risk. Also, carpool lane is on the right on some expressways. A special situation is the 2-lane part of highway 5 between SF and LA. Sometimes one truck needs to pass another, causing traffic in the left lane during the pass. Courteous drivers basically form a line in the left lane waiting for this. A few people pass this entire line on the right then cut in.