Because the bezel on the backlight part is exactly the same as before. This means the screen can't go right to the edge. It's also the same dimensions as before, so the design can't be changing drastically, like on some of the mockups.
I guess it's possible they could do something interesting like integrating the home button into the screen with the current screen dimensions, but that would mean the phone itself would get short and fat, which seems unlikely.
I imagine the design of the 7 will be very similar to the 6s, just thinner again, perhaps with the antenna lines removed and maybe some new camera feature, more memory or something similar, to convince people to part with their cash again... just like the last few times.
The iPhone 8 is more likely to be bezel-free, once they start using new OLED screen tech.
