Question: mounted guns destroyed by artillery

[Imp]

Thats because its a soft skinned target so it sounds correct to me. Bits fly through its thin skin or canvas & kill the "soft' people huddled together inside. Most times if its not a near hit this causes little damage to the tough part of the vehicle, chasis & engine so it can at least keep going.
Same principle as riddling a car with SMG fire, it might stop eventually if the radiator is hit otherwise it wont have much effect unless you get a lucky shot or of course take the best option & try aiming for the driver.