Tenacity, Magic Resist, and what you are neglecting

This ends up overlapping with the all assassin meta. More specifically, the meta that the damage dealers should deal nothing but damage no matter what. I kept telling the assassins and the bruisers, the guys who are dealing all the damage, the guys being focused with crowd control, the one thing that screws them over that prevents them from securing that one kill, from pursuing the carry and finishing what they started:

Tenacity. Tenacity. Tenacity. Tenacity. Quicksilver. Magic resist. Nullify what the supports can do to you.

I don't know what's so difficult about one item that also provides damage and magic resist AND the ability to cleanse all crowd control on you IMMEDIATELY that no one seems to understand even exists in the game. In one game several days ago, I was able to solo a fed Tryndamere as SUPPORT LULU and win. CONSISTENTLY. Why? I kept my distance, I slowed him when he tried to get close, I cast whimsy on him so he couldn't even fight, and then peppered him down to nothingness, even forcing him to burn through his ult. This happened half a dozen times throughout the game, and never once did he build magic resist or a scimitar or tenacity or anything to try and defend himself from me or cancel out what I could do. No, he'd rather build more critical damage, because he REALLY needed that Phantom dancer to cover up that last 5% of critical chance, otherwise what's the point of living?

It continues even into premade games where I'm in a four man queue. Three of us are literally nothing but damage; slows are an absolute premium. Nobody builds anything in the sense of defense. I tell them repeatedly, endlessly, to build tenacity or a scimitar or SOMETHING to stop the enemy from locking them down while their allies escape with just a sliver of HP. No, why build something that could negate a snare when you could get an item that makes you kill faster?

The League community right now is focused too heavily on pure offense, that the only crowd control worthwhile is a kill, and the only items are critical chance and damage. Everything else is the support's job.