Runaan's on Cait?

Every ADC benefits from runnans. Other items are just better.

While it does stack her passive, the fact is that any AA-focused ADC centers their builds around damage & Critical stats because they give the most damage potential. An item that gives purely AS is simply underwhelming in terms of stats, and the passive hits it grants are 1/2 damage and circumstantial. An item that gives crit stats will always be better in the long run.

Stacks passive faster

I believe it helps stacking with her passive, but I've never seen it build seriously on her.

I believe the passive on Runaans has a lower range than her AA. Not an ADC main though, don't know the ins and outs of it.

crit chance from shiv or PD is better

It does proc her passive which can be useful but there are several better items for her.

Makes you wonder why items like this and Rageblade exist in the game if it is never a good idea to build them, as in it's always better to build something else (aside from teemo and kayle on the runnan's)

Unless your name is Kalista please don't

Cait is one of the 5 champs on which its not a total troll pick.

But theres something better in most cases