In every movie except the first one (where society was starting to collapse, it hadn't fallen yet) the question of "where's the gas" is a major plot device

I'm not going to sit here and argue Beyond Thunderdome is high art or anything, but kind of the whole point of the Mad Max films is not just to comment on resource scarcity and control but to make them the call to action for everything in the story. You're asking "who's making all the gas" because that's exactly what George Miller wants you to be thinking about.