The "obvious" problem is harder than it looks. Even searching in the navigation app gets confused by voice prompts from the same app...so you get things like "find closest gas speed station check ahead". And that's before you get various people involved, never mind any impediments.
And speach is not as easy as a button / knobb for muscle memory. In my old car I didn't have to think, just do. A touchscreen just can't compete in a car.
