Armor vs MR disparity

Why is MR is strong while armor is kinda weak? As a tank to take care of one fed AP hero all i need to do is buy one MR item. For a fed AD, well i need two and that in itself isn't adequate. I played a few AD assassins quiet recently and i think Lethality this cheap is kinda OP? Even when the opponent still has an armor item it's quiet easy to assasinate them. Magic pen for mages isn't even that cheap. Anyone feel the same?