I'm guessing it's because that type of shadow can't cast onto dynamic models (i.e. the weapon in your hand) except as a primitive form of ambient lighting change around the player, which is more of a trigger than the actual shadow. RAGE did that, and it felt very 2003 in what should have been a cutting-edge engine. It also precludes the use of a convincing day-night cycle in a game, if that's something you want to add.