Because a higher standard of living requires more people to come up with the said standard of living (see: history, the shifting of innovation centers to population centers)

The degrowth movements garden variety neo-pastoralist misanthropy makes it blind to the fact that more humans (given a good system of governance and rules) is actually an amazing thing.

I hope you do accept there are physical limits to growth that there is no way get around short of colonizing other planets and that we should at least take care of our mother planet.