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, others 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 tries to pass as slow as possible because they are going a few mph over the speed limit.

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 many people are just never consider that others also need to use the roads. They don’t care about traffic. Some people also have health issues, like blindness, or mental handicaps, which means driving at interstate speeds is about all they can muster.