Because the situation didnt present itself. I cant imagine that pilots weren't allowed to do whatever they could to have any kind of advantage on the enemy. I very much doubt that a plane like the P51D or Ta152 with their tail heaviness under full fuel load (aux tank in the 51, aft in the 152) wouldnt have been prefered in a DT+ minimum aux/aft tank weight if the situation would have allowed it, as is the case in AH. The P51's aux tank IIRC was explicitly refered to as a liability in aerobatics, if not an outright disqualification for it.
I cant see why any pilot in his right mind would opt for something detrimental to his plane's maneuverability that he couldn't ditch, as opposed to the same thing but detachable a few buttons/switches away.
Conversly, there's no reason to have B25s with fake defensive guns take off anywhere in the game, because the situation doesnt warrant it.. And it's laughable to replace those guns with extra fuel in the situations in AH.