Google Maps is a good general maps app, but Waze is really good at one thing only: driving. In Waze you can share your drive, look for gas stations along the way (with prices and travel time), use toll passes, and get a personalized ETA based on your driving.
Google has had the chance to implement these features into Maps for the 10 years it has owned Wazd but just hasn't, and I don't think they will. At least not all of them.
You can do all these in Maps (minus toll passes). I'm not sure how an ETA could be personalized (I don't use Waze) but Maps is pretty accurate with ETAs as is.
In Germany for example there are large sections of the highway with no enforced speed limit. Depending on the traffic, I can easily speed with 200 km/h. Do I? Well, that will depend. At least in the past I did. So the prognosis when I will arrive on a route with mostly highways would be highly different for me than for someone who prefers to drive 120 km/h.
Obviously the other way around is the same: even when 130 km/h are the allowed maximum, I could legally drive 100km/h (or even less). If the software calculated my time of arrival by assuming I will speed to the maximum, it will be wrong.