Google has made it very clear that they want to build Google Maps for iOS and provide the same experience as is available on Android. It was, for a long time, an explicit no-no to provide turn-by-turn navigation in an iOS app.
Google doesn’t allow third party developers (and that includes Apple, obviously) to offer turn-by-turn navigation. Here are the terms (10.2 c): https://developers.google.com/maps/terms
Letting Google build the maps app is obviously out of the question. That’s a pretty ridiculous idea. Would Google let Apple build a critical piece of Android? Obviously not.
(Also, where has Google made that clear? That’s a serious question. I’m not aware of any such statements from Google, so it would be awesome if you could point me in their direction.)