Several factors. Prairie chickens and buffalo which were primary food sources are all but extinct. People don't have to knowledge to harvest edible wilds and they aren't accustomed to eating the foods they'd need to eat. You'd have to spend about 16 hrs a day harvesting, preparing and eating enough cambium to survive. Your stomach isn't large enough to do this. Most of the things the natives ate are extremely low in nutrition by today's standards and you'd find most of them distasteful to the point of being inedible. The people who try living off the land experiments (such as Zachary Fowler and Wooded Beardsman on jewtube) typically lose about half a pound a day, which is totally unsustainable even though they are in optimal environments. They typically are on a lake or river and during the few weeks of the year where berries are ripe and edible. I'm not aware of anyone who has tried to live off the land for several weeks who hasn't lost 20+lb. Settlers would typically carry a significant amount of high calorie food with them, such as hard tack and supplement it with the wilds they found.