I had wondered about that before. My guess is that Google considers Waze and Maps users to be different type of "navigators", with Waze being a self-selecting group of folks who want crazy routes to shave a few seconds here and there, while the Maps users are more mainstream and just want a sensible route with options. So they may have hesitated to give the "crazy" option to them.

Another thing, I suspect there's a significant resource cost in constantly re-evaluating these few-second saving opportunities that may not scale well to the size of Map's user base. Could be wrong here.