Why is it they can build so many defensive items yet still do big damage it makes 0 sense

someone please explain to me why the enemy team built almost entirely defensive and still did so much damage. At one point in the game the zoe with 3 defensive items still was able to blow through me and i have sources of mr. why are defensive items so powerful steraks gage gives big defensive shield and tenacity but why does it give you so much ad as part of the passive why does zhonyas give so much ap as a defensive item and banshees. along with the actives and passives of the items. someone please explain.