Most cars were never intended to be without a protective layer of paint. For steel cars, once oxidation starts, it spreads a decay and associated ugliness that few enthusiasts can stomach (although many can tolerate, provided it doesn't prevent them continuing to use their vehicle). If it comes from within, by the time it shows itself, it's already too late...
A failure of seam sealant allowed corrosion to start between panels and work it's way out here. Not pretty.
Of course, some people deliberately provoke light surface rust, to add patina or character to their cars. It's not my scene, but I can appreciate it on some vehicles.
Of course, the exception to all this is when cars have bodies made of aluminium alloy (or aluminum for Americans and people for whom 4 syllables is quite enough). Most aluminium-bodied cars are still painted, to provide a more consistent finish and conceal the panel joins, but those whose paint is removed (or was never applied in the first place) have a certain character and attractiveness which rust-prone steel cars can never attain.
When aircraft started to use aluminium alloys in their construction prior to World War 2, they were often polished bare metal to demonstrate the new material and it's innate attractiveness. The same could be said of many cars whose creators and owners took advantage of the additional bling for only a little extra effort. However, the more pragmatic racing car constructors would often forgoe the extra weight of paint and do without the polishing part too.
1938 GN Spider II at Shelsley Walsh Hillclimb (although clearly in the process of descending after it's run)
Some cars could be ordered without paint, others had it removed afterwards. All of them are beautiful in their own way.