Innate heals for tanks

Why is giving tanks big innate heals, the ability to do a bunch of damage, good CC, AND while simultaneously letting them be impossible to bring down when they inevitably get on you a good idea? I know there are ways to kill tanks but how is giving them all the good and none of the bad(Volibear for example) a great idea? Voli has tons of movement speed, tons of damage and huge tank stats. No way to get away from him, No way to kill him without at least 3 people. Tanks are stupid OP now.