The problem is that fighters/bruisers are hard to balance. Give them a good defensive item and they become OP (or tanks become OP, because they can utilize the item better than bruisers). Give them a good offensive item and Marksmans/Assassins become OP.
Let say Riot make a good item that gives health, AD and armor. If the item isn't strong enought, it won't do anything and if the item is strong enough to replace another AD item (let's say TBC) It becomes abusable by other classes (marksmans, assassins), since they scale better with AD and the survivability will help them even more in order to survive.
I wish they do something, but at the same time, I understand why they don't do anything about it.