Why is it that tanks can deal so much damage?

No specific champion, but champs like Shen, Poppy, Illaoi, all deal fucking insane amounts of damage without a single damage item in their build. How can you kill/beat a champion like that?

I just come up against an enemy Shen and he absolutely carried the shite out of their team. We couldn't do a thing about it. We could win a fight 3 death for 4 kills, and shen would just solo the remaining two champions no matter what hp they were at. If we focus him to kill him, then we have nothing for their semi-fed diana mid, and can't get to the back at their cait. We also have a thresh to try and get past who will just shove away anyone trying to get to cait. What can I do in those situations where a super damaging tank is carrying the game?