Tanks and their items are better at stopping AD than AP.

I am tired of being blown up so easily by AP champs despite having two MR defense items and lots of health. Meanwhile, if I go armor, it's very difficult for enemies to kill me due to armor items having better effects, such as returning magic damage per hit with thornmail or slowing attack speed with warden's mail. There is literally no comparative item when it comes to magic items, component or full. I cannot increase the cooldown of enemy spamming abilities nor can I silence them (only champ that can is Chogath and at short range), nor can I even drain mana (you use to be able to with old Veigar or with the old Wit's End). The only exception is Adaptive Helm, and I've still been melted by champions due to just sheer raw AP power compared to my measely 150 or so MR.

Tanks are only good at being anti AD in this day and age. There is a reason that Riot has nerfed every anti-mage counter in this game. They want mages to be able to run rampant and do easy damage. You cannot tank magic damage anymore. You have to suck it up or hope some hard CC or assassin kills the mage.

Mages are the Gods of League of Legends due to this damage - defense disparity that comes with them. They also have the most accessible items when it comes to killing tanks:

I repeat: TANKS ARE NOT ANTI-MAGE ANYMORE.