You're missing a lot of context here.
The big movement against silences wasn't about the silence mechanic itself, but rather the fact that Assassins were utilizing silences. An Assassin jumping on you should be a time of fight or flight response kicking into overdrive and you, as a squishy for example, doing everything in your power to not die. However, counter play becomes meaningless when an assassin has access to a silence because, and this is not a hyperbole, they can silence you, burst you down, and there was absolutely nothing you can do to stop it. Not to mention that these silences given to assassins were all targeted abilities.
Knock Ups and Knock Backs on the other hand tend to either be extremely short in duration compared to their tenacity affected brethren (2.5 second silences, 3 second snares, 3.5 second stuns, etc.), so there's that. For the next part, the amount of targeted knock ups in the game is so much proportionally lower than targeted cc of other forms. "Fully uncountable" knock ups? Rek'Sai needs to be sitting under you, Kalista needs her Soulbound and can be incredibly slow to set up, Azir needs you to be standing between him and a soldier on top of putting him in melee range thereafter, Gnar needs to be in essentially melee range, Braum needs to either be in melee range or he needs to hit you with a relatively slow skillshot, Vel'Koz telegraphs, Yasuo needs to charge his via multiple usages of a nearly melee ability, Aatrox telegraphs, Zac's puts him in melee range, the only "Uncounterable" knock up is Quinn's fricking vault in this long line of knock ups, and hers lasts an instant. To call these uncounterable is an insult to anybody who knows that dodging abilities tend to counter their effects.
