This kind of isolation is very interesting to me. There is a lot of space out there, and there are probably a lot of acres that only get one human visitor every 5 years. I'd give myself a 1% chance of noticing the rifle if I walked past, based on this picture. So it's not at all surprising that the last time anyone looked at this tree was in the 19th century.
That region of the US, which is maybe half the size of Germany, has been largely devoid of human habitation since the neolithic Lake Lahontan civilization disappeared a few thousand years ago. It is difficult to overstate just how vast and uninhabited that area is.
When I was in my 20s, some friends and I would make long treks into the deep off-road parts of it to explore it precisely because so few people had been there in modern times. Back then, we made our way into areas where we were probably the first humans to come through since the 19th century. The area is actually littered with archaeological artifacts that remain undisturbed. Those were fun times.
Highway 50, which runs near there, has been dubbed the Loneliest Road in America (http://ponyexpressnevada.com/highway50/index2.html) and that's a relatively populated corridor in that area with small towns "only" 50+ miles apart. Great Basin in a national park but doesn't get a lot of visitors relatively speaking and most of those are either going to the caves or the summit of Wheeler Peak.
The areas right next to parking lots in National Parks are very busy, but usually once you get a mile down a trail, even in a very popular park, it's very nearly empty.
It does vary. The most popular National Parks certainly have their busy spots even a mile away from the road. But your basic point that people on/near road >> people on any trail >> people a few miles from road is absolutely true. Even somewhere like Yosemite, you can easily get away from the crush in the Valley and, especially with backpacking, can get away from most people all together.
