Our economic systems only work with infinite growth because otherwise what would be the point of lending money if it won’t grow interest. It’s essentially a giant pyramid scheme. And that requires new blood to provide labour and consumers. This is incredibly dumb on a finite planet with limited resources, but that’s mainstream economics for you.

Also if the population shrinks too fast, then the pyramid becomes unstable with not enough younger people to take care of all the old people (while also maintaining the economy).