If Camille is in this mode, why can't Riven be in it too?

because then riot would have to realize how strong riven is when she builds full lethality

Too short of CDTs for her to be even remotely healthy since she also has a crap ton of hard CC and a shield which scales off of AD.

Riven is stronger than most assassins when put against most assassins. Lol. Riot knows this.