Hypothesis: population growth looks like /, while public land growth looks like - or \ (e.g. cursory search for statistic shows federal acreage drop of 4.9% from 1990 to 2018)
I'm not sure where you live, but try googling about the terms "free camping" or "dispersed camping" for your area. There are lots of natural areas (edit: in a lot of places) where you don't need a special permit to pitch your tent.