Being in nature is good for us, so that’s probably part of it, as is the constant practice of being self-reliant.
Another theory is friction. I can’t just roll out of bed to the couch, and back. I don’t have a fixed place to shower, so I swim in a river most days, and usually it’s freezing. Camping alone was terrifying at first, but objectively not that dangerous and not worth worrying about. All these little moments are a chance to exercise that mental muscle to do uncomfortable things.
Modern life is devoid of this friction. I could try to take a cold shower every day, but a not-cold one is just a turn of the wrist away. I could try to make myself go outside a bit every day, but I don’t need to.
This isn’t a new observation, but I can’t believe how stark the contrast is. I’m getting stuff done that I’ve been procrastinating on for years.