It's probably coded into the OS to not display that option if Windows detects the capacitive buttons on the device already. Same thing for the 5th toggle in the action center on the Lumia 1520 vs smaller devices like the 1020 that only have 4 - There is logic in place in the OS to do things based on the kind of hardware the phone has
That would be the "back arrow-home screen-search" buttons at the bottom edge of your device.Does the 1520 have the navigation bar? I can't find it on mine.