That's because Human nature will make us form societies, groups, nations and install law and order in some manner. We are a social animal, arguably the MOST social animal on earth, to the extent where we thrive and need other Humans to survive and live full lives. Even in times of great stress and problems Humans will band together to SOLVE the problem, not merely tolerate it. We Humans strive not merely to survive but to CONTROL the environment and the resources in it. That's why Fallout 1 makes infinitely more sense because it takes into account the fact that Humans will strive together to prosper, to rebuild, to make something more.
Basically, what I'm saying is that Humans are naturally inclined to forming societies and rebuilding, not acting as insane drug addicts and rampaging around fucking over people. Humans are dicks, selfish, cruel, corrupt, etc but they get things done.