Eh. . . you're thinking about this far too simplistically, as what you said doesn't make a whole lot of sense.
Right, so take two theoretical champions, A and B. We'll ignore resistances, attack speed and abilities for right now, and just look at it in terms of health and damage, with them attacking each other at the same speed.
Say A is the tank, let's give them 2,000 hp at 18, and 100 damage. B is a carry, we'll give them 1,000 hp at 18, but 200 damage.
If A and B both attack each other at the same time, they will kill each other in 10 attacks, and both will die at the same time-- perfectly balanced.
But, let's add (overly simplified) items, what if A gets a defensive item, and now has 2,500 hp? Ceteris paribus, A will win every time, as after 10 attacks, B is dead, but A still has 500 hp. By buying defense, they were able to win the 1 vs 1 that would otherwise be a tie.
The same is true in League. Defensive purchases add just as much potential to a 1 vs 1 as offense does theoretically.~ If you increase your defense by 1%, that will counteract them increasing their offense by 1% exactly. So if a tank gets ahead, and can increase their defense by a higher percentage than the carry or assassin player increased their offense (while also not increasing their own survivability AT ALL something which used to be report worth and considered trolling in just about every season before S6) then why wouldn't everyone expect the tank to win that 1 vs 1?
If it takes a minute for you to kill me, but 30 seconds for me to kill you, who do you think will win that fight? Why is that a surprise, and why are you framing this as some evil, abhorrent abortion of game design?
"How dare those dumb, skillless tank players kill me, a glorious and amazingly talented rightclickr, or edgy, amazing azzazzin weabo who is so inherently superior to these shitlords, whilst they didn't even deign to purchase a single offensive item as a pure defensive class who gets less from offensive items than literally anyone else! The blasphemy! The sheer nerve!" sneers away into the distance
~or at least, if they were perfectly balanced against each other it would. As it is the scaling is vastly different, so ceteris paribus offense is always strictly better than defense; additionally, there is far more armor/mr pen than AS/AD reductions, no crit chance reduction, %hp damage, true damage, sustain based off of damage [
Ravenous Hunter, oh and any form of lifesteal/spellvamp] so, long story short, defense is actual, literal shit compared to offense and almost never worth building despite the fact that it should be equally valuable in part because ADC and Assassin mains have bitched and moaned up a storm to riot each and any time, well, 'a tank beat me without any offensive items OMFGWTFBBQ RITO pls NERF NAOW!@!!@!11!!!' or, well, in this specific case:
"The problem is right now a tank building no damage items can 1v1 a lot of champions who are building damage."