There's cooldown, attack speed, armor, magic resist, magic pen, swifty's but no Armor Pen. Why? Some people like Lucian, Ezreal and Jhin could benefit from the armor penetration more then attack speed. Any reason why? Even 10% would be nice.
Crett6 votesCheap Armor Pen for assassins sounds like it could go very wrong
Variks the Loyal4 votesBecause it's already on a lot of items. 
Compared to Magic Pen, which is on half as many. 
Anraton1 votesyou missed 
EDIT:
Fun Fact: Magic pen on is not unique :-D
EDIT2:
Fun Fact 2:
+ + = 40 Armor Pen
+ = 30 Mpen + 20 AOE from - up to 50
is more or less equal with 
Zero Skill Tank1 votesIt is also noteworthy that squishier champions have a lot more difficult time itemizing for armor than for MR, and that HP counters typical AP carry somewhat harder than an AD champion.
Anraton1 voteswell there are certain items that just dont exist .. like armor + damage
don't get me wrong, i understand your issue but i think some missing items are design decisions
