The Android back button is a concept that doesn't really exist on iOS (except as an informal protocol with inconsistent UI cobbled together between a handful of apps, many of which were written by Google...): swiping left is more like clicking "parent folder" than "back"; think what happens in a file system or web browser when you click a symlink/shortcut or hyperlink: that is closer to the Activity/Intent model of Android, and back serves the universal purpose of not just going "up" the folder/breadcrumb trail inside of an application and an actual "back" in Safari, but also allows for global "back" during tasks like returning to the web browser from an app you might have entered, or back to Yelp from the Maps app, or back to YouTube from a web browser.
It's not the same thing as the back navigation on Android.
Also I find swiping very imprecise compared to a statically located back-button near the bottom of the screen. Apps implement swiping differently. For some of them we have to go all the way to the edge of the screen to swipe and for others you do not. Swiping is also a completely different experience on the iPad compared to an iPhone.