Two things here explain this for me. One someone already mentioned I don't believe Max is one person I think he is a legendary figure that gets merged into one person as people talk about their local heroes. The other is I always viewed the first movie as one of the holdouts of old civilization. For whatever reason that region had the resources to be in a more normal state for longer. When Max fucks off into the desert he's going deeper into areas that are more desperate and have been hit harder by everything. We don't know the full landscape of everything. The bat shit stuff we seen in later movies could be relatively isolated even but the society that does remain could be more like city states that dont have the power to go in and control the wild areas.