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There are actually 6 armor pen items but 2 have the same passive.
Armor pen: 
Magic Pen: 
Didnt think about it much until recently but magic pen has much less variety.
Armor pen has options. Options are always good. Id like to see 2 or so more made next season but make them all share a unique passive so you cant stack maybe? idk what do you guys think?
Because 
also increase magic damage dealt
And it's a bit disingenuous to list both last whisper upgrades as two seperate items when they're built mutually exclusively. Wit's End does the same job as Black Cleaver but also missing from your list for some reason.
Less magic resist items in the game? And the magic pen items do a lot AND with deathcap you can multiple that ap damage by a bunch.
Well 3 of them are assassin items while pretty much ever ap champ can build all magic pen items. Tho Id like it if Void was split into 2 unique items like Last Whisper
Natural MR is lower than natural Armor. Simple.
This is why no one takes mages seriously, its like they dont play the game or something. How much base mr do people have at max level? Search it up plz
and
AP abilties have higher base damages and scaling and are cast much more frequent than AD abilties.
Before you mention Zed, understand that he is 1 of like 7 AD ability-casting type champs vs the 50+ AP ability caster types.
i'll ask a few very simple questions. What is the LOWEST base MR in the game at lvl 18? now tell me that value for armor (excluding 
). Does that answer your question as to why there are more items to deal with armor than with MR?
actually 6
