People like anthropomorphising. I think the various bigfoot type stories have the same explanation as faries, elves and all the other magic humanoid stories that exist throughout the world. For pretty much all of history, every time something happened, it was given an explanation that involved humans. Sometimes they were big humans that lived in the sky and dropped thunderbolts, sometimes they were little humans with wings, sometimes they were friendly, sometimes they played tricks and so on. The only real difference with bigfoots is that magic and faries aren't generally taken seriously any more, but we since we know apes do exist, that seems an acceptable explanation to some people, even though they are never actually supported by the facts.