Tank stats don't matter unless you're a top laner, jungler, or pretty much any role but support.

I'm a support main and here lately ranked has been frustrating the absolute crap out of me. One game, an illaoi without a single true tank item (cleaver, boots of swiftness, and trinity force), became so ridiculously tanky that she was impossible to shut down, even when her ult was down. The game before that it was a swain, even as a thresh with close to 200 stacks, eye of the equinox, ninja tabi, banshee's veil, spirits visage, and Warden's mail (was working on randuins), i didn't come even close to tanking the amount that he did, with his build of a rod of ages, zhonyas hourglass, abyssal septor, and rylais septor. This happens all the time, i pick a tanky support, trying to work with my teams comp, and i just end up getting shredded mercilessly unless my entire team is ahead. If a top laner is behind, he can build tanky and remain relevant late game. This doesn't seem to work with supports. At all. Despite the math adding up to me having far, far more hp, armor, and magic resist (not to mention flat damage reduction from bond of stone and ninja tabi's passive), and a full set of tank runes (+16 armor and magic resist), and masteries, they still end up being far far more tanky than i am. What is this logic? what is this math? Does riot have a secret hidden algorithm that takes any support laner in draft pick and cuts their arm and mr effectiveness down to 1/3? it certainly feels that way.