Because a) It would be boring and b) less tanks were knocked by airpower than claimed by the airforce. According to Niklas Zetterlings book Normady 1944 only about 7% of destroyed german tanks during the Normandy campaign (circa 1500 in total) can be said to have been knocked by airpower.
In Sledgehammer by Christopher Willbeck he explores the knocked Tigers during operation Goodwood. 13 were lost to airpower, 7 of them by high altitude carpet bombers before Goodwood began. Typhoons knocked 6 of them.
During the entire Operation goodwood the two airwings claimed some 390 knocked tanks. After the battle some 460 heavy German AFVs were standing knocked out on the battlefield, 300 were inspected. 10 had been knocked by airlaunched rockets. 3% of claimed became confirmed.
At Mortain the airforce of the allies claimed about 75 more kills than the germans had tanks.
Basically this myth about rockets from P-47s and Typhoons being the bane of German tankers is nothing more than a myth. AT-guns did the most of the damage both in the east and west.
There is your explanation, it doesn't make sense form a gameplay standpoint nor from an historical one.
Why you speak about efficiency? Do not confuse efficiency with the fact.
In 1944 german dont have fuel, factories did not produce enough tanks and tanks had technical problems. Most trained soldiers were already dead or in captivity.
Myth? This game is full of myth. Blitz in 1944, Obersoldaten (what is this? zombies on steroids from Wolfenstain? Sturmtiger and Ostwind (zero impact in war and only few pieces).