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.

https://www.mdpi.com/2076-3417/11/9/4278

TLDR: modeling traffic as a gas leads to fairly accurate predictions. If that doesnt mean anything to you, here's a decent visualization of how gasses move around in a system. In this analogy, each of the gas particles models a car on the road. https://youtu.be/Hr5Baj3lXFA

You are thr traffic.

Years ago I was in LA on business and ended up on I-5. All of a sudden traffic stopped dead. Eventually people were getting out of their cars and walking around. About a half-hour later, traffic started up again. We never saw what was responsible for the blockage. No wreckage, no obvious marks or debris on the pavement, nothing. I'm glad I live in a small city nowhere near California.

George Carlin explains it pretty simply

https://m.youtube.com/watch?v=XWPCE2tTLZQ

basically it's a thing with cars where if a single car slows down for any reason, even slightly, it causes a cascade effect that leads to traffic jams.

Yet another reason why cars suck.

Bumper riders Lane switchers Brake checkers Ramp users switching lanes wayyy too late