Stacking too much armor is counterintuitive because a large portion of it's gold efficiency is rendered obsolete as soon as one of the sustained-damage AD chanps on the other team builds a LW item/BC and starts ignoring a large percentage of it. Armor also has diminishing returns depending on how much you buy so it naturally gets less gold efficient as you stack more.
Not as big an issue on bruisery Champs with %HP damage/tanks with high base damage values/tanks that don't need to do much damage at all......but consider if you were heading toplane with a different tank or fighter and had to 1v1 against a competent player and every single morsel of gold is of significance! How are you suppoed to make your trades do damage if you spent all your gold on mitigating damage? And what are you going to do when they start ignoring that damage mitigation and, god forbid, they start to outscale you?
Armor is a useful Stat, but it's possible to have too much of a good thing. It really is a balancing act.