Hmmm, not the best analogy. I agree about metal needing to be maintained, its not that hard. But if you put metal away and not come back to it for years, you can easily find corrosion, even on blued metal.
Exhaust systems, the biggest thing that makes a car exhaust corrode is the heat it is exposed to. Put your pistol in an oven to heat it up to 600°F - 350°F several times a day for as long as you drive your car that day, you'll find it is corroding very quickly over a year or two.
Heat and temperature will always speed up a chemical reaction, and that is what corrosion is, a chemical reaction with the metal and oxygen.
And you do see machine gun barrels that get much hotter in use than most firearms, and if not maintained properly, do tend to corrode faster/more than other firearms or parts.
Yes the exhaust has water in it that can get into the exhaust. It as much getting the engine warm enough to get the exhaust warm enough that the water stays as steam and doesn't condense as liquid water, and the exhaust system will cause the exhaust to cool as before it leaves. And stainless steel exhaust will use different alloys of stainless steel, some rust faster than others. And the design of the exhaust itself will dictate how hot it will get and how much water it will trap, as well as the engine it is installed on. So it just turns out some combinations will corrode much faster than others due to a combination of a bunch of variables, and I haven't even gotten into road salt, regardless if you get it warm enough to evaporate the liquid water out or not.
Stacking of Tolerances plays into that as well, multiple parts designed to fit together, if you're unlucky enough to have two parts at the limit of their tolerance in opposite directions, then that adds up for being out of tolerance overall. Or the term is used in design, that they design for stacking tolerances so that even in the worse case scenario of tolerances for individual parts, won't put it out of tolerance overall after being assembled.
