Because its generally not needed. Going back is exclusively an internal App function and as such is implemented within the App by its developers as required by the app context, and current situation.
The iPhone has been a single button device since its inception over 10 years ago. (and now removed from the iPhone X) Its flow and design was explicitly conceived as such.
If you require a physical back button, then you are doing something wrong. The App and its flow should provide for a way to go back within the App if there is somewhere to go back to.
Not sure how imaginative it would be to have the same button layout as every other smartphone out there.
As to the keyboard, it has no arrow keys period. If you need to move the cursor around there are ways to do this, a back button is not one of them.
As said, perhaps dropping the preconceptions of Android and learning how the iPhone does things instead of expecting it to work like an Android device would be a better option.