Because our economy is somewhat of a Ponzi scheme, in which everything needs to keep growing forever. We are perpetually borrowing from the future, with the ass

Because our economy is somewhat of a Ponzi scheme, in which everything needs to keep growing forever. We are perpetually borrowing from the future, with the assumption that everything will be growing forever. As people get older, they start getting more expensive. If you don't have a larger generation coming from behind to provide for that ever-increasing older generation, then the costs have to be spread among fewer people.