All those materials with the exception of animal furs are commonly found on or in the ground. Given the relative lack of multistory buildings throughout our history, that's basically just making the ground a few inches taller. The ground itself is highly varied as well, with various combinations of rock, dirt, sand, etc.

I was thinking more that in the developed world most people have much more separation from anything resembling the ground, both in terms of materials and physical distance that would substantially alter any electrical characteristics.

In the off chance I win the lottery I intend to fund serious robust studies of minor stuff like this. Would be the easiest placebo ever, just internally disconnect some of the mats.