Anti-CC options Vs anti-mobility options
The game has four options for shutting down CC that any champion can pick up (

)
The game has no options for shutting down mobility that any champion can pick up.
Targeted CC is the only real champion-specific means of shutting down mobility, but such effects keep getting removed in favor of skillshots, which in turn favor mobile champions who can better position themselves to aim/dodge them.
So why not an item that shuts down mobility as hard as Quicksilver Sash shuts down CC? A targeted active that reduces the range all Flash/Dash effects to zero for a given duration. So Kass could still use his Ult, he just wouldn't go anywhere. Kalista's hops would do her no good. Riven would have to walk into melee range. Vayne' Tumble would leave her in the same spot. Ect, ect.