Because a society in which everything that needs to be done is getting done - is stagnating. A progressive society always discovers (or invents!) new things needing to be done. And somebody needs to do those things.
Is that really the worst thing in the world? In an age driven by hyper consumerism and buying new shiny iPhones and other gadgets, do we really need to do more? Can't we just be happy with our accomplishments as a human race and just enjoy? I for one would welcome a stagnant society.
Also, humanity's eggs are currently all on one basket - we're one major cataclismic event from complete obliteration.
No, the challenges ahead require us to keep evolving, we cannot afford to stagnate.
Or some machine...