Riot only likes balancing champions that make flashy and interesting plays,riot could care less about non mobile-non high risk champions like Zilean.
I'm not sure "like" or "dislike" really come into play with Riot. As a successful company, they're more interested in making objective decisions over subjective ones.
And generally speaking - there main objective is making the game feel like it has solid design and gameplay, so they promote champions with solid counter-play, aesthetics, play patterns, and gameply - while downplaying champions lacking counter-play, riddled with anti-patterns, and bad gameplay.
Regardless of how annoying it is Zed has been a top-tier champion for several seasons, he's still a really solidly designed champion.
As for Riven and Vayne - it's important to consider the game is balanced with the competitive level in mind. Those champions simply aren't amazing picks at the competitive level, nobody's really going to ban them out or pick them as a top priority.
All they really have is an unusually high amount of fame from walloping scrub-lords. They aren't even exactly good at winning vs. scrubs with ~50% win rates, they just hog a lot of glory by winning via getting kills rather than controlling objectives, zonning, sieging, or solid team-fights.
Finally, a 99% movement speed boost isn't exactly not mobility in regards to Zilean. I wouldn't really call him immobile.
With the range of his bombs and what his ult does, I'd call him low risk rather than the alternative as well.