The balance comes when the tools immobile champions have are able to compensate for their lack of mobility.
For example, when Xerath's Stun and his Slow can make a champion think twice about engaging on him, then the mobility is ballanced. However, when those tools cease to be a consideration, then the ballance is skewed. What does a Kalista care if she gets slowed? Nothing. She doesn't need movespeed, and she has enough dashes to likely avoid the Stun too. Riven is in much the same boat, able to jump in or out of danger as needed. These two are by no means the only ones who can ignore slows, and this in turn leaves every champion with a slow in a weaker position than they otherwise would be.
We've also got mobile champions that can reduce immobile champions mobility even further. Tristana just got a slow on her Rocket Jump, Riven has a stun, Azir has a knockup with his dash, Kalista has a slow, Lee Sin has a slow, Kassadin has a slow, Gnarhas a slow and a stun, etc. Note that (with the possible exception of Azir) there's absolutely nothing the immobile champion can do to avoid the insuing CC. Contrast this with LeBlanc, who you can try and stun or zone away in order to not get rooted.
Right now we're at a point where the mobile champions hold all the cards. There's no reason to pick immoble Champion A when you could pick mobile Champion B who has the same CC, same damage, and more ways to avoid danger/engage. Either the balance team doesn't know how to give an actual cost for mobility, or the cost they are giving mobile champions is too trivial for the benefits mobility gives.