I am surprised Waymo did not try to target this market first as its a lot lower risk and easier to pull off.
It's amazing how many people bought into the hype that there would be widespread self-driving taxi services by the end of last decade and that kids growing up today would never need to learn to drive. Even a relative skeptic like Rodney Brooks is looking to be overly optimistic at this point. https://rodneybrooks.com/my-dated-predictions/
Probably the same reason Google kills projects if they don't become a giant market leading success quickly. At Google/Alphabet's current scale a moderate win is not worth pursuing.
The real question here though is "Is this safer and cheaper than just paying a bus driver $12/hr?". My suspicion is no. And when you don't have the scale effects of millions of applications, it simply doesn't make sense.