Of course they have downtime. They need to charge. Waymo partners with car rental companies for depot operations where they inspect for damage and clean the cars. Damaged cars need to get repaired.

But, a Waymo car has a huge impact compared to gig drivers. Even ones that do it 12 hours a day. The Waymo fleet is maximally available for both rush-hour busy times plus closing time at bars and nightclubs.

I don't know if they've released updated fleet statistics, but in the past I've been shocked at how small it is relative to the visibility and passenger miles. This indicates that downtime isn't a limiting factor.

Gig drivers optimize, or try to, their individual revenue and other preferences the shape their responsiveness to customer demand. In other words it's two sided market among individuals. Waymo isn't. Waymo optimizes across their whole fleet for revenue, presumably, and customer satisfaction.