Very simple. Because there are real-time conditions on the road not known by Google Maps. Google Maps can tell me to turn right at an intersection, but I can se

Very simple. Because there are real-time conditions on the road not known by Google Maps. Google Maps can tell me to turn right at an intersection, but I can see that police cars have blocked that road so I decide to proceed straight. Same thing with Waymo; its routing indicates turning right but it finds obstacles on the right. So it asks a remote human what to do. Both of these are "where to drive" questions. It's just that their answers come from different data sources, and one is easier to answer than the other.