The target zones of military artillery ranges, to name a silly example.
The military aren't out there all the time. When they're not playing, and the red flags aren't flying, those training areas are good for walking, and sometimes more.
There are also plenty of upland areas where people only ever traverse a few fairly narrow paths, there being next to no reason to go tramping about on trackless upland moorland.
I guess you've never been a lover of the Great Outdoors? Some of us go out there precisely to get away from the beaten track, though sadly that's not really possible in Blighty.
But I have no expectation of a phone signal when up on the moors. If I do get one, it's a bonus.