I think Apple made the right call here for novice users. The back button on Android is useful, but it's very mysterious. You use it by kind of mashing it and hoping it does what you want, and then finding another route if that doesn't work. That's fine for power users, but it really adds stress for novice users. Novice users tend to have low lying fear that they will mess something up, and they need to understand things before they do them. Back button can never be understood, even by advanced users, because every app implements it differently.

There's nothing preventing every app on iOS from implementing a back button, but they have to label it, and it has to stay in-app. This forces power users to do the work to navigate around app boundaries, which can feel clunky, but it makes the interaction model much more compressible so it fits in more brains.