Why true damage?

It's something that has been on my mind for awhile now. Riot has made plenty of items designed for tanks. Tanks build items centered around the idea of surviving and these items give either health, armor, and/or magic resist. Usually each item only gives a maximum of bonuses to two of those things. If a tank wants, they can focus on just one aspect of defense, get armor for an AD heavy team, get MR for a magic damage heavy team, and then they get health to sort of help them against those that deal damage that they are not built to sustain.

But then there is True Damage. A damage type that the only thing you can do to build against it is to just get a shit ton of health....it just seems counter productive, especially when True damage is somewhat common among champions considering that almost every champion will either have a true damage ability, have ignite, or will build a true damage item.

So essentially the only way to truly build a tank is to just build a ton of health meaning that all those items that give armor or magic resist are kinda pointless.