[2021] Why is Riot nerfing champions like Skarner, Rammus, and Udyr instead of Turbo Chemtank?

I posed this question to r/junglemains yesterday and someone suggested I post to the main subreddit too.

When 4 of the 6 highest winrate champions (Udyr, Skarner, Rammus, Mundo) are all building the same item, isn’t it obvious the issue isn’t the champions, but the item? I feel like these champs are going to keep being nerfed to a point where chemtank is the only buildable option since their power has to be balanced with this item in mind and every other mythic is going to feel terrible just to make chemtank feel balanced on them.

I do believe that some of these champs (mainly Udyr) should receive a nerf along with chemtank, but if Riot’s goal is to promote item diversity, then why are they nerfing all of these champions around chemtank instead of chemtank around the champions?