Why has Riven not been nerfed?

because she's not op.

So you decapitate Kha'zix 60 times, change Rengar. Yet, you leave Riven untouched.

Riven is the number one champion to be called busted and overpowered on the forums, yet she sees almost no changes. The last change was her Valor shield duration, which isn't even a big deal. The problem with Riven is her intense mobility that was unrivaled for a long time, and her power to drop people 100% to 0 with the slightest of ease. You simply cannot justify nerfing Kha'zix and Rengar, but not touching Riven.