Serious question. Why doesn't cleanse/tenacity work on knock ups?

Well think about it like this. There is soft CC, which are slows and debuffs. There is Hard CC being Snares and Stuns. Then there is True CC which would be Knockups. If people could just stack Tenacity then tanks would be even harder to deal with, or fighters to that extent.

Because that's the defining attribute of knockups.

You might well ask why Armor doesn't reduce True Damage.