Magic penetration item deficiency
Lets just compare the number of items there is for both MR and Mpen with numbers of Armor and Apen items(summoners rift only):
MR:
Tank:
AD:
AP:
Supp:
+

Armor:
Tank:
AD:
AP:
Supp:
+
MPEN:
Flat:
%:
APEN:
Flat:
%:
+
So with 14 MR and 16 Armor items one would think there would be around the same amount of Mpen and Apen items...but one would be wrong. There is only 3 Mpen items of which one is the boots and there is 6 Apen items. While you could see most ads,fighters and assassins building these Apen items conditionally since each one provides unique advantage over the other. almost every mage goes into a build of: mana item > morello/defensive item> the one not picked previously > rabadon/void staff> the one not picked previously- adding boots along the way; with the exception of the tanky mages going into the dps line of lyandry,rylai etc. but even those would get morello and sorc shoes in a lot of the cases so...
Why are these items so limited to a point they stand in the way of any build creation which doesn't involve them? Also...Is it fair for most AD champions to have the choice of modifying their builds according to what fits the best, while AP champions are left with only one option if need arises?
/
gives more defenses than
, which are the cheapest respective flat pen items, provide flat pen