Why? Conventional car body is steel, which rusts easily. So it's (most likely) galvanized + protected by various layers of paint.
If that paint is damaged (scratch, dent, whatever) the underlying steel will rust. Repair may be along the lines of "sand down, apply primer, apply color, apply clearcoat".
Stainless steel is different. The base material itself doesn't corrode (as) easily. Similar to say, aluminium, titanium or even copper. Repair: sand down until scratch / ugly spot is gone, sand down with increasingly finer grit, finish with polishing material (wool or whatever), done.
There is a catch: some conditions may corrode even stainless steel. A common culprit is salt(s). Especially repeated wetting / drying cycles where droplets of salt water concentrate, potentially causing so-called "pitting". Various grades of stainless have various degrees of resistance to this.
According to this article: (thx Wikipedia!)
https://www.motortrend.com/news/tesla-cybertruck-electric-pi...
The Cybertruck uses 301 alloy stainless? Perhaps the thought was "if it's good enough for rockets, it's good enough for a car". But... exactly that may not be so!
It's not hard to imagine road or environmental conditions that don't apply (significantly) to rockets: ocean breeze containing salty droplets, winter road conditions (including de-icing salts), small stones hitting a car's underside, etc etc. Also 1.4~3 mm. thickness is more easily compromised than whatever thickness a rocket's hull is.
304 stainless is common (and good enough for maaany applications). 301 is probably similar. But in eg. marine applications, 316 (or 316L) alloy is common.
So maybe Tesla goofed (or penny-pinched?) on their choice of alloy? Or they didn't, and this is a non-issue blown out of proportion? Who knows. I wouldn't worry about it unless dipping-in-salty-water is regular pasttime.