Why does society value growth above all else?

Because a bird in the hand is worth two in the bush. I'm not particularly driven to accumulate wealth, but if my income doesn't keep pace with inflation my purchasing power will diminish, and if I don't save then I have no hedge against the possibility of unemployment or inability to work.

now, you might argue that things don't grow forever in nature, and you'd be right. But the natural world is not just about beautiful vistas and flowers every spring. From bacteria to blue whales, competition in nature is intense and unforgiving, and even dominant populations are subject to occasional catastrophic busts. Left to itself (as in a few unusual environmental oases) a population will expand to use all the available resources, even if the individual members of the population are limited by growth. Organisms generally grow to the extent that they have a good chance of reproduction. they will evolve through successive generations in whatever direction maximizes reproductive fitness.

Schumacher was an interesting theorist, but (like many other idealists) his vision of an ideal world depends on mutual abandonment of competition. I'm a big fan of mutuality, but there are limits to its scalability.