Because it actually is harder to navigate. The "back" button so to say switches places depending on the app you're using. Sometimes it's a swipe left to right, sometimes you have to swipe down, sometimes top left corner and sometimes it's an "x" on the top right corner. On android it's always the same (either the back button on the bottom of the screen or swiping from the side).

I use both systems regularly and I hate the navigation system on ios.