How do modern cars regulate temperature so well?

Because back in those days, the car companies gave you what you got... They weren't trying too hard to give us the best that they could do...

It wasn't until the Japs came in and started taking over the market, that the OEM's started trying harder to give us better products...

The foreign car companies came in and started doing it better, so the OEM's had to step up their game or loose their business...
Consumer Reports always lists American cars at the bottom. In recent years, after the bailout, some are up higher.