What if there was a tank item that removed 45% of an opponent's bonus AD

Yep. What if there was an item that made me a pizza and slapped a bronze player every time he told someone better how to play?

You make that sound so ridiculous, but that is exactly what buying armor does. It even counts base ad as well!

If AD and armor scaling were somewhat one to one when it comes to strength then your argument would be pretty darn good.

Problem is they are not.

I would laugh so hard if that was a thing. And then I would play nothing but tanks.

Doesn't Frozen Heart reduce physical damage by about 45%

You mean what armor already does? Yeah, great times.

and then you play tryndamere

Yes it does.

DONKEY! thats so Ridonculouos! Says Shrek

For most of the champions (read: tanks) that build Omen or Frozen Heart, they have a lot more in their natural kit than just defense to take down ADCs and assassins; the tank items help them survive long enough to make the killing blow. Health, Armor and crowd control already achieves that; coming from someone who gives the assassins and ADCs endless amounts of shit, they need the edge up on tanks with Last Whisper.