My vehicle has a weather map but it doesn’t overlay with navigation. Seems like a simple benefit that would get people to switch their nav service to using whoever implements it first.
People don’t pay enough attention driving as it is constantly looking at their phones, adding tiny icons and lines to mark weather systems would be just another thing to distract already terrible drivers.
If you want to look at the weather then pull over and do so, I don’t think it has any place as additional distractions in a vehicle imo.
MyRadarPro has a travel mode and works with CarPlay. You can give it your destination and it will correlate travel time to future forecast. It will tell you things like, it will be raining when you reach this point along your route.
Open Street maps has weather data, though not a real time overlay
Apple Maps already shows weather conditions, temperature, and air quality in the bottom right when using the app (might need to check Settings as I don’t know if it’s in by default).
But while driving / with a destination set, the interface is far more minimal to avoid distractions.
You can always say “Hey Siri, what’s the weather like in Cummertrees?” or wherever you’re going.