I thought it was basically animals brought up to sapience trying to keep the very small number of humans left alive comfortable and happy. It's basically a conservation effort. They realized we re-wired our brains to require us to be working off debt to be "comfortable" so Tom Nook pretends to loan us a bunch of money we have to work off, and that's also why the economy makes no sense and everyone is just so happy that we sell them a potato or a nice rock or whatever.