Google would have no need to buy Lyft. Google Maps has a much bigger install base and already has ride share integration. Lyft does have technologies that would be useful like matching riders and drivers and Lyft Line. Creating that back end is not trivial task for Google but it not a 20 billion dollar one either.
Waymo is going to buy and retrofit and provide 1.4 million cars?
or areas/weather conditions which they know are difficult for their autonomous cars to navigate.
If they bought Lyft, they wouldn't have to do that all at once; they could just insert self-driving cars as they were available into the pool.
Optimistically with economies of scale you can get that down, but even without it's within the realm of possibility if they've got the technology working.
2) 1 human driver != 1 autonomous car. While cars need servicing, they don't get hungry or tired or have a family to spend time with