Not trying to just nitpick, but if we assume that the Google Maps use research and data from users when planning features then the feature you're after may actually not be very common.

And your answer to someone asking "Why do you think that use case is that common?", your first line literally just talks about your use case from your point of view:

> This is exactly what _I_ usually want to do with maps. _I_ have a route _I_ want to plan, and _I_ want to do more than one thing along my route, or see what else is in the area. It's futile in Google Maps.

I'm not saying that wouldn't be useful, it's just that maybe not that many people need it... I guess it was built with the idea that you would just open more tabs to search other things?