Google's built-in app (Duo) is made available on iOS, but Facetime isn't made available on Android. Apple could easily open up Facetime and iMessage, and they'

Google's built-in app (Duo) is made available on iOS, but Facetime isn't made available on Android. Apple could easily open up Facetime and iMessage, and they've evaluated doing exactly this, but they ultimately determined that keeping these pieces of communication software exclusive to Apple devices is a big part of why the devices themselves are so sticky.