Because then it would affect a lot of other champions, and not their specific target audience.
Example:
Very theoretical example, not true but could work, replace champions and items to cycle.
Let's say
is completely abusing Lich Bane, you have two options; nerf the item or the champion. But if you nerf the item then here's what happens.
Who else builds Lich Bane? That's right,
and the occasional
. Now if those 4 or 5 are sporting sub 50% winrates, and use Lich Bane as a crutch. Then they will be indirectly nerfed and they will eventually need to be rebuffed. (And knowing Riot, they will take their merry time) Just like how right now, Udyr is making things quite miserable for Elise and Diana JG due to the RE nerfs that were practically specifically targeted toward him. Moreso for Elise because she's very reliant on Runic Echoes RN. Neither of those will recieve buffs however, because Elise is still very pops in competetive, and most Dianas go solo lane anyway.