Its because of two things. One is that people hog the passing lane or try to pass slowly so it takes them a few minutes to overtake a few cars, and also because people drive at different speeds. Some people drive at the speed that feels comfortable, orhers drive the state imposed speed limit. This creates pockets of dense traffic, and then people try to pass, but there is always the person who fries to pass as slow as possible because they are going a few mtoh over the speed limit already.

Its really just a bad combination of laws, and drivers who are terrified of breaking the law, and people who dont know how to drive correctly in a way to reduce traffic. Also mamy people are just never consider that others also need to use the roads.