Friendly fire

Not sure if this has already been asked, but why doesn't arty/direct air attacks damage all units that are engaged in combat? I feel like this a fair way to nerf the current dominance of arty, as rushing forces would cause the opponent to consider an increase in damage, or to cease bombardment.

Things that comes to mind is what the soviets did in Stalingrad, where they hugged the german line so they couldn't get picked off by german bombardment and artillery, without the germans inflicting massive self casualties.