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.
I am exactly one of those users which you describe and only use Waze for long trips and Maps for the rest. Waze urban navigation drives me crazy with the route it picks sometimes, hoping of shaving off a few seconds.
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.