I fast for 12 hours frequently before working out, & I mean weight training, not cardio. It feels like I actually have *more* strength when I do my lifting fasted, however, I feel like I also burn out harder.
I think you're right about the availability of food in the early environment, however, I think the part that people fail to connect with these days is hunger. I think if you have an otherwise normal healthy person who gets hungry, they will get the idea that there's something wrong with them. So people nowadays (by which I mean nearly everyone) seldom go more than 8 hours (for sleep) without eating.