I don't know for sure that it doesn't change, however I'm almost certain that it can't be changed by anything the manager can do so that is why it isn't shown. All other skills can be changed via what the manager decides. For example training JS will increase JS and the secondaries. And simply putting a player on the court increases experience. But I doubt that playing extra or less minutes has any effect on the aggressiveness of the player.
My other reasoning would be that aggressiveness is meant to simulate the personality of the player, and just like in real life, personality doesn't really change (obviously there are exceptions in real life but I'm talking in general).
Also, my old PF that I'm sure you remember (Edward weird last name here) was a very aggressive player and constantly got 5 or 6 fouls a match, and he was a really old player, so either he was really really really aggressive when he was younger (which I doubt), or his aggressiveness stayed the same throughout his career hence why he was still aggressive at an old age.
But as I said before, I'm not 100% sure and never seen a BB confirm it so this is all just a theory and speculation.