It's much more expensive for the people to replace their cars every two-three years because rust spots develop everywhere on the bodywork.
Chemistry is a very wide area and the manufacturers are obliged to research solutions - elements which don't react with water/salt.
No car is going rusty after 2-3 years. It's not car makers fault if you are buying old bangers, and they are certainly not obliged to do anything beyond what is required to back up their bodywork warranty.


