I'm curious, why can't tenacity reduce the duration of knock-ups?

A knock up is essentially the same principle as a stun, you are locked in place and unable to initiate attacks, however one you visually fly up into the air before plopping back down and the other you just... stand still. Two champions benefit out of this difference, everyone's favorite 1-13-2 samurai, and "this is why I carry bear spray when I go hiking" thunder bear. I'm going to make two small assumptions here why knock back is irreducible by tenacity and stuns are not.

1. It will look ridiculous and disorientate players if their very visually attractive knock ups become minor bunny hops
2. The two aforementioned champions will suffer from reduced windows of active exploitation.

However, now we have another champion who gets an active bonus off of CC'ed enemies... and not just knocked up ones. Swain can right click any CC'ed enemies to pull them towards his plasma hand of annihilation. Tenacity already effects some of the CC, but knock ups are all the same duration, regardless, meaning if someone buys tenacity and the primary source of CC applicable for swain to benefit off of does not include Knock ups, he is viable to miss time pulls because it wears off. He is already vulnerable to this window of timing error, why not just make it universal and generalize tenacity to effect knock ups too?

I believe some of the benefits to this is an effective buff to bruisers(whom to my knowledge need some edge) but also allows for more adaptive game play rather than playing by timing sense.

Also, I want to clarify now that I am not addressing knock BACKS, which I think are perfectly fine being irreducible because reducing them means either champions fly shorter distances or they fly faster, which can screw combos over or make them a more deadly Lee sin cannon ball.