Why is there no armor equivalent of maw of malimortus?

cause the game is already broken enough as is, adding more problems to the mix is ill-advised

Maw is good because it is being used to survive the burst of mage champions. In general, ad champions have sustained damage (because their auto attacks hurt) so a shield that blocks burst is far less useful. That being said, Stereks works as an anti burst item with a shield that stops physical damage.

With assassins on the rise, however, its possible that an ad version of Maw will show up eventually; its just the case that ad assassins haven't been strong enough as a class to justify an item devoted to countering them.

There shouldn't be a armor version of Maw... Though there should possibly be an armor version of Banshee as there aren't many tools for squishy champions to deal with physical burst damage