Magic resist items have better passives and stats than armor items.

Notice all of the armor items are purely defensive while are all offensive. Not in the fact that "you need to fight" but reduces enemies MR while being close to them, When it gives you life steal promoting that you continue fighting, increases ALL healing which is pretty busted when you think about it, and which gets better the more you fight. The only offensive armor item MIGHT be provided that you are chancing the enemy down. Also there is one more MR item than there is armor. Clearly Riot hates magic dealing champions.