Because Gran turismo and Forza motorsport have been banned from being able to put Porsche cars in their game.. and supposedly it was EA who banned it.. why do EA have the right to ban Porsche from car games?
More likely it was Porsche who may have withheld permission to depict their cars in the games. Other than that kind of conflict, a game producer can choose whatever they want in their games. In other words, it isn't "banned," but simply not chosen for whatever reason.