I thought this as well, but I think it emerged in an Waymo interview that any weird thing that can happen on city street can also happen on a freeway and the reaction time is lower and the consequences higher on a freeway.
Freeways have some of the same challenges as streets, but not all of them. Treating the environment as if anything can happen at any time is just an "abundance of caution" thing. To use a real example, you don't get people walking up and throwing an egg at the side of your vehicle on controlled access freeways.