I'm not sure why you would say there's no significant inclement weather in Atlanta. The flooding this week was not super common, but also not unheard of. It rains here a LOT in the summer
The part of that people aren't considering is that it's very common to get brief, intense thunderstorms that dump a lot of rain quickly. They won't flood the whole city obviously but there's _always_ pockets that have very short-lived, localized flooding on the roads. So it's not a "oh what are the odds of that happening" kind of thing.
Agreed, this happens here every year, it's why we built O4W park the way it is, and built many other drainage structures similarly. We have a real runoff problem. Waymo picked a great city to train the cars on weird weather and weirder roads. :D