Well, technically they can, it's just that that sort of lifestyle is unrecognizable to us today, and would basically take all your time. They would be totally left behind by modern society, ie hermits.
I mean you're right in a unabomber type sense (i.e. living in a log cabin trapping small animals), but if I wanted to build my own smartphone from scratch, my whole lifetime is spent extracting materials to build the equipment required for making the smartphone. Even if I somehow made the smartphone, I would have immediately violated some patent law.
Even if I were a globe trotting immortal, I cannot legally DIY most common electronics because of patent law.
No one can DIY everything.