(Math incoming, TL;DR at the bottom)
For this example, we're going to just be looking at armor/ad, but the principles can apply to mr as well.
The gold value of armor is 20g per 1 point, and health is 2.6g per one point. Attack damage is 30g per 1 point. It _sounds_ like the defensive stats should be more efficient, right?
Here's the problem. High-end armor items vs high end AD items, AD gets a huge kick to efficiency that armor doesn't have. Let's look at some of the usual suspects
Bloodthirster: 80 AD, 20% Lifesteal, 3500 gold, 101% gold efficient w/o passive
Ravenous Hydra: 80 AD, 12% lifesteal, 100% hp regen, 101% gold efficient without its passives
Rageblade: 25 AD, 25 AP, 25% attack speed, 3300 gold. 73% gold efficient without passive, jumps to 125% gold efficiency at 6 stacks, further increased by 17% for every 100 AD and 11% for every 100 AP.
Youmuu's ghostblade: 2900 gold, 55 AD, 10% cdr. 90% gold efficient without passive/active, 106% gold efficient while out of combat and 133% gold efficient while using active.
Now compare with high-end armor/hp items.
Sunfire cape: 2900 gold, 60 armor, 425 hp. 80% gold efficient without passive.
Randuin's omen: 2900 gold, 400 hp, 60 armor. 78% gold efficient without its passive or active.
Thornmail: 2900 gold, 80 armor, 250 hp. 74.71% gold efficient without passives
The numbers are clear and easy to understand. Damage is far, far more gold efficient, and far more slot efficient.
**But wait, there's more**
1) Running conqueror can reduce the gold efficiency of all armor/mr items by a flat 20%
2)Hp burn drastically reduces the efficiency of building any form of hp
3)There's more true damage in the game now than there's ever been before
4)Damage helps secure objectives far more effectively than defense does
5)Damage items have actives which are far more effective than defensive items
6)Armor shred is more popular than ever, as well as armor pen
7)Damage users can itemize damage and increase their effectivity against everyone. Tanks have to itemize specifically armor or MR to increase their effectivity against only one or the other.
Defensive items are simply inferior to offensive items at the moment. This disbalance needs to be fixed.