Because every other skill can be improved or declined. Aggressiveness can't change, so it isn't shown.
That´s so not true.
Height is shown, cannot change, but highly influences your plans with the dude.
Potential is shown, cannot change, but highly influences your plans with the dude.
I just don´t understand why some characteristics of a player are shown, and some are not. Whatever influences the player´s developement, it should be shown. Or at least mentioned in the manual.
There´s no plausible point treating potential or aggressiveness differently.