I only play really tanky champions (mainly support) and whenever the other team has a yi or tryndamere (or both) I always build thornmail AND Randuins. Yet that does nothing to stop them. I still die within 2 seconds. So whats the point of building tank if it barely helps at all?
This is complicated to answer.
Most tank choices are on their face, by themselves, weaker in exchanges than building counter damage. (most pros will tell you this).
Most tanks don't build tank items by themselves. They have kit mechanics which make them more useful than the base item.
(For example, Second wind is exponentially better of a rune the higher your HP goes, and works even more powerfully when you have Overgrowth. Therefore, these two runes dramatically enhance the usefulness of having armor/HP items, because the armor will reduce the damage take, while Second Wind will heal the damage that WAS taken)
The champs you're talking about currently are doing a ton of true damage, so while Second wind helps, The armor helps far less. If they have Conquerer, it's best to make sure you have a CC or dash left to escape. Let them do their signature skill, and then cc them to peel.
Since you're talking tank supports, ever think about 
? Just knock them back off of you and ruin their ults, and don't face tank. Thresh and Blitz are bad picks because if they miss, they are chowder.. Taric can shut down both with his ult, and stuns, if played well.
Its only because of this retarded rune and guinsoo, like 2 days ago i saw master yi killing braum with 3 aa wtf XD
as a tank its your job to be the focus ... if youre an alistar for example youd want to go in being tanky and just disruppt and annoy or being that frontline for your team ... being that guy who wants to take damage.... if you die so what as long as your carries can easily kill the enemy team then its for a good cause :)
A year or so back they nerfed tank items and buffed tank champion innate defense. Problem is that Runes Reforged came in and skyrocketed damage across the board for everyone. This did a few things:
Overall, I feel like this 1-2 punch was the worst thing that's ever happened to League as it's turned everything into a damage arms-race, shifting the game into a "battle royale" mode and much further away from having a more tactical, measured element. Long team fights and chases were always my favorite aspect of the game and now it's just "fight to the death, winning team takes 1-2 objectives, repeat" which is incredibly unsatisfying.
Well I mean what else are you going to build on braum haha.
I understand what you are saying because IMO building defensive on an offensive champion is actually dumb and never really seems stall your inevitable death like in the past. (except in certain situations where hexdrinker and wits end are OP)
Edit - oh yea and braum just got his defenses pretty heavily nerfed so you might be feeling that aswell.
tanks were op in pro play (still are) so they had to make them shit in soloq
If that does nothing to stop them, the problem is on you, not the items.
The answer is because you have nothing else to build because Tank supports and most tank champions have worse than shit ap scaling for having 300 less range than mages.
Try not building tank against them and see how it goes. It may seem like your tank items don't do anything, but if you were to have no defensive items you would get instantly deleted. Living for 2 seconds is better than .25 imo.
Yi is a tank killer, and so is tryndamere cuz of conq with his kit makes him one now.
