Why Isn't QSS it's own item?

Why hasn't riot made QSS into its own item that gives something like magic resist and health or Magic resist and Armor? Even if they didn't change it they should at least make an ap item out of it. The active is way too good to be something that builds into another item, I know they have Mikaels but that's just a crappier QSS that doesn't do as much. Even LCS pro's when they need an extra cleanse on Azir or something build QSS just for the active effect.

TL;DR Title says it all.