Because Android features this metaphor, it's used extensively. Because iOS doesn't, it isn't as much. iOS features different metaphors which achieve the same outcomes in different ways, such as pop-up sheets for the camera, album views and share panes.

And in cases where one app does activate another, you can go back one step by tapping the top-left corner of the screen. There's only one level of "app back", but that's plenty since iOS doesn't tend to jump you between apps a lot, and because that navigation isn't co-mingled with other "back" operations.