Armor feels absolutely useless in comparison to MR

Anytime I lane against an ad mid I always rush a Seekers into a Zhonyas, which feels worthless. I really don't see a difference. Disregarding the Zhonya active, I feel like armor is useless against people like Zed, Talon, Yasuo, etc. Yet all they need is a and my entire damage potential seems to be gone. I feel like this item hasn't become an item that you build against heavy ap teams, you just build it anyway. It gives you armor pen! Why is it that ad's can find so many ways of building armor pen (and even if they won't they still hit like a truck) yet mages only have (lets not even talk about how cost-inefficient these items are, if they all build mr and have no magic damage, you still buy abysall even though the 50 mr are worthless here, and so many mages don't even need liandrys passive, you just buy it for the 15 magic pen sometimes). Void and sorcs are the only non-situational magic pen items. Abysall has mr and liandrys passive is suited for very few mages. We need another magic pen item and we need to buff void staff into its prime back. Im sick of facing ad mids when I know they will RUSH a hexdrinker even if im behind just because it provides you with so many stats.