"We want items to be equally efficient"

I guess it's because armor protects against more things like turrets, auto attacks, physical damage dealing spells, jungle creeps, minions, baron. Meanwhile Magic resist is only useful against baron, magic damage dealing spells, and magic damage dealing on hits. Armor is way more useful so it costs more

Armor ≠ Magic Resist

EDIT: Just to be clear, you are comparing apples to oranges.