It really isn't. We're just utterly and contemptible shite at sustainability and efficient use of resources. How much food do you think the west especially wastes every year? How much water? How many consumer products are built deliberately to be cheap and disposible, with absolutely no attempt at making things last or indeed any interest in repairing things rather than simply throwing them away and replacing them?
The world could sustain a hugely increased population over what we have today, but at some point it's going to require some basic resource management and allocation, rather than just the consumption free for all that we're all so used to.