The only way a door gets left open is if someone orders a ride and leaves it open. That person is charged a fee, which pays for the Doordash person - which is cheaper than installing motors/automatic doors in every ride. And, there's good availability of Doordash (or Uber Eats, etc.) drivers in every market Waymo is in right now.
Why isn't the loud audible warning? Maybe send notification? It seems insane to charge for this instead of adding big flashing warning light for example.
The car announces on the outside speaker that the door isn't closed, turns its hazard lights on, and they send an app notification. Some people will still miss all of the above, or ignore it. Sometimes the seatbelt is jammed and stops the door from closing, and they assume the car is 'wrong' and walk off. There will always be edge cases, you build operational policies like the doordash one to cover them.
Seems like a pretty good solution, honestly.
Yes, it has cost, but this is a solved problem.