What about the "wild man" theory?
That bigfoot is basically inbred humans who live on the outskirts of society because they are... well... kind of messed up.
This theory would explain why there is no large breeding population that we can find, and also why bigfoot is seen in every culture.
So family has sex with themselves out on a remote farm untill.....
There are a number of problems with this theory from an anthropological perspective.
One, where are the tools? Genus
Homo is identified in part by our tool use: stone axes, control of fire, cooked food, to list the most basic features since the first species identified as
Homo, H. habilis lived some 2 million years ago. If BF is genus
Homo, it must use tools of some kind. Where are these? Where are the remains of the ancient tools, and where are the examples of new ones? (Paleontology and archeology are thriving fields in NA, BTW. How could professional, college-funded diggers, whose careers are based in making new and important discoveries, miss what must be a massive load of such evidence?)
Two, how do we explain BF's body hair? Genus
Homo is identified in part by a lack of thick body hair and the presence of sweat glands, which we evolved (the current thinking goes) in order to withstand longer durations in the sun, traveling and hunting on the East African savanna. In short, an animal with such full body hair or fur as BF is reported to have would not be classified as genus
Homo.
Three, in order to remain a genetically viable species, the minimum population of this animal would have to number in the thousands. This counters the contention that "there is no large breeding population that we can find".
Four, if "bigfoot is seen in every culture" (which isn't correct to begin with), are you suggesting that this same hairy phenotype has branched off coincidentally from the main population in every region of the earth where humans reside? What would be the mechanism by which this astounding event would have occurred?
Five: "So family has sex with themselves out on a remote farm untill...." What farm? A bigfoot farm? Where are these farms? What are you talking about? This theory is ridiculous and I'm now ashamed I addressed it so seriously.