It's not a zero-sum game because debt is limitless. Imagine an extreme example where you rigged the game and literally won all the money in the world and then kept winning, what would happen next? People would find another form of payment via services and the money is worthless if you are the only one who has it. Therefore you might lend them money in exchange for a service so that they will keep playing your rigged game now accruing a credit of services in addition to money.