Why do champs get to build all tank and still do full damage?

This really confuses me to be completely honest. A lee Sin can go full tank and still almost one shot targets. How on earth is that fair? If you build all tank items then that is what you should be, a tank. Not a damage dealer. It boggles my mind because it almost makes it seem like the items don't even matter. It's just who can play the champion that has the most broken abilities. Like with the whole issue that fizz had when he could just build straight tank items and still get a pentakill. shit like that needs to be fixed in this game and I honestly think they have just chosen to ignore lee sin in that issue.