Opposite to the claims seen elsewhere in this thread, which are omnipresent whenever the topic arises, it is in fact humans who are driving around on easy mode and self-driving car developers operate in the most difficult conditions they can find.
You might be confusing them with Cruise. Waymo is driving around SF driverless 24/7 so the million miles also come from during the day, while Cruise is limited to nighttime.
Ehhhhhhh. While I agree that these aren't the easiest driving environments in the world, they are certainly mild climates. If I were in the business of proving that waymo vehicles weren't ready for prime time I'd be taking them up the alcan in winter or Florida during hurricane season, not Phoenix.