if defense were bad then we wouldn't have an issue with things like aftershock mages and kaisa making you kill her 3 times before she dies for real
what makes tanks mediocre in terms of actually tanking is that the only thing they get through their items is adding onto their immediate defense. this is perfectly fine for general purposes, since tanks are generally there to open up the fight, not see it to the end. And it works fine in pro play too because they can coordinate and turn that initial tank boost into a close-out. Not so much for soloQ.
What makes fighters better at surviving fights is that while they have less immediate defense, they have sustain. They can *return* their defenses back to them consistently assuming they get over their vulnerability to burst focus at the start of a fight. This is the biggest thing that separates their strength from tanks -- sustain is just much better than raw defenses at pretty much every point after Aftershock wears off. Because after a certain point the rank items are only valuable for their passives, and all you're doing with the stats from them is just making enemy counter build more efficient.
The sustain gap has gotten a lot wider, too. Tanks used to have pretty generous amounts of health regen back when it was more readily available and was flat pen not % based. This made Visage a pretty valuable item for tanks even if they didn't have innate healing, because the boost to their regen was good enough. Also we had Banshees and GA as tank items. GA is important to note because it essentially gave tanks a "free" 30% of their health back, so you could technically call it sustain. HP regen right now is only available on an item most tanks don't buy and which makes you free food to anyone with good % damage. So, that compounding effect effectively removes a large part of your survivability because you're losing sustain and not compensating that elsewhere.
Basically, if you want tanks buffed as *tank* threats and not *damage* threats, don't ask for armor buffs. Ask for health gains back.