Because there were no supermarkets. When they ran out of food, they had to go look for more. When they found it, they had no major reason to go back where they came from, since there were no mortgages, office buildings, elementary schools, high school football teams, churches, shoe stores, or radio stations.
Early humans did not raise crops or animals for food. They picked fruit, berries, leaves and roots that grew wild near where they lived, and hunted animals for meat. If they picked and hunted faster than food could replenish itself, then the supply ran low, and they simply picked up and went looking for a place that had more.