In real life warfare, gaining air superiority is very important and often a deciding factor.
However, in Warhammer 40k, most ground combat is in very close range, but still all participants can freely airdrop soldiers, vehicles or other material. Actually, it is a perpetual war and the ground is a big meat-grinder, while reinforcements are constantly brought in by the millions from off-world. Do their dropships just peacefully coexist in the air or in orbit? Maybe it's just a simplification, and there is a war raging in the air, but the players are leading only the ground combat. In this case, how can it be that no one ever achieves air superiority and denies the other parties their dropships/bombardment/whatever coming from above?
I understand that this is for providing a competitive balance for the players, but was this ever addressed in canon?