Armor scale modelers love to cover mufflers and exhaust pipes of their models in significant layers of rust. But the question is: would they really rust so badly? If in beginning they were painted just like the rest of a vehicle, we need to allow some time for the paint to pill of. Now, a deep rust will not set in in a matter of a day, that may take a while too. Not to mention that military vehicles were constantly cleaned and repainted. On top of that we can add a short life span of those vehicles as well (they may not even survive long enough for rust to appear).
So the question is: would those exhausts rust as bad as we try to present them? Maybe they were just dirtier, but not rustier?