Why is Google still insisting (with 7.1) that the 3 navigation keys (menu, home and back) need to be on screen - either wasting real-estate or annoyingly hidden - requiring a swipe to expose them?

Old Android phones used to have hardware or software keys that were off the bottom of the screen - and in fact my Xiaomi Mi Note Pro still does - it frees the entire screen for content. If the '3 keys' regularly changed meaning I could see the need to have them 'on screen' but they don't.