It was later, when humanity accumulate knowledge about resources gathering and processing, about nature and how to deal with it to not to die all the time. Then yeas, tribes were becoming larger, wealthier, more stationary. But before that there were very few people, the tribes were nomadic with virtually no alternatives and had nothing of value. At least nothing so valuable that it would be easier to get it by attacking another tribe, rather than by simply moving a couple of dozen kilometers away.