It shows up on social media when it’s a rare event for that area. It’s uncommon but “happens all the time” here in California in the deserts every heavy rain either because locals forget how deep the flood control washes are, or because tourists just drive into them thinking its a straight road, despite all the signs and warnings posted around them.

As far as I can tell from these articles, driving into a flood has happened twice to Waymos, once in Texas and once in Atlanta? It does seem like it's pretty uncommon.