In most places, because the economy needs to grow so it stays ahead of its growing loans and debt (overly simplified). To grow, you need more workers and customers. If population doesn’t grow, and you don’t have immigrants to do the producing and buying instead, things stagnate, very lower interest rates that the system can’t really handle, government keeping the economy together with duct tape, general welfare not doing great what leads to even less population growth. But every place is a bit different and its own challenges of course.