well, historically it was because manufacturers didn't have the opportunity to sell six-figure cars, so they had to use the cheapest acceptable steel alloys in order to produce a product that people could afford -- and paint was a 'decent enough' band-aid to the corrosion problems that came directly as a result from using cheap and cheap-to-manipulate alloys.

i'm not a Tesla fan, but let's not be ridiculous and presume that corrosion-resistant stainless steel is a myth; it's abundant on sea vehicles. Tesla decided to develop their own alloy and it back-fired in their face; this isn't a testament to stainless steel itself but rather to the intricacies of metallurgy and Tesla's own NIH-syndrome.

later deloreans used good-ole 316 Stainless rather than Tesla brand "Ultra Hard 30X stainless steel" and they still look great many decades later.