And tanks have a very, very hard time hitting choppers. They are fast enough to dodge the tank shells as they go past, and they can attack them from a high altitude, decreasing very much the chance a tank shell will hit them. Take into account that you have to aim higher than the chopper is to get the shell to go where you want it to, and a tank can't fire straight up, so if chopper was hovering over it, the tank couldn't hit it at all.