Once you are in the Waymo, why bother getting out just to get on a bus?
In New York, increases in the use of ride services like Uber mostly came from bus and subway riders[1]. If we are being charitable to Google, we might assume that this initiative is intended to reduce the same sort of trend. If we are not being charitable, we might assume that this initiative is intended to encourage current bus riders to expose themselves to the product.
[1] https://www.6sqft.com/mta-says-uber-use-is-the-cause-of-nyc-...
It makes a lot more sense for Bart. You can go places that don’t even have Waymo service.
The answer to both of your points is that mass transit is used for most of the travel and the last mile is taxi equivalent. Being unable to do the last mile is a common reason not to take mass transit, so more usable taxi equivalents tends to increase mass transit use. And most directly, taxi equivalents are often significantly more expensive or significantly than mass transit at crossing cities.