That's not why cars don't rust as much. Coatings have just gotten better and simulations have gotten better so engineers can know in advance that all the salty water that runs down the vehicle will be channeled through a few spots that will rust out in short order then do it anyway.

Galvanization was popular for a brief time while the industry was still trying to find replacements for lead in paint that didn't have crap performance.

Cold galvanization is expensive and involves nasty chemicals. Hot galvanization is cheaper but not as cheap as paint and involves equally nasty chemicals and molten metals.

Basically paint is cheap and good enough.