You suggested this thinking one particular occasion but there are other situations where you'd be hitting a vehicle constantly such as DB or spraying your Uzis all over the vehicle, which happen more frequently than sniping. In those scenarios the hitsound will be too annoying. Also if this would be implemented you wouldn't be able to understand if you're just hitting the vehicle or actually damaging the player inside the vehicle, which is imo more important. That's why I'm against this suggestion.
-1