Tanks are wrecking face because no one plays against them properly.

Just had an enemy Poppy destroy my team and get all of her team ahead. Not once did anyone buy any sort of armor/magic pen besides sorc boots, outside of our Wukong who never once fought the Poppy anyway.

Before that I had a game where I was a nigh-unkillable Taric and our enemy never thought to get any sort of armor/magic pen until the late game, and it was the Yasuo with a Mortal Reminder. By then we'd already taken almost everything they'd had so, the chance to keep me from being effective was already long gone.

In fact, most games where the tank goes absolutely crazy literally are only occurring because people are a) buying no pen so the tank does their job even better, b) buy no resistances so the tank "does too much damage", or c) both of the above.