Because they keep doing shit like this. NeoMalthusianism 101: You can't expand infinitely within a finite space. Earth is a semi-closed system and, as the Millenium Ecosystem Assessment reported, many of the planet's habitats are already degraded, or threatened with degraation by human activity. And the obssession with growth- both in economic and human population terms- means that that degradation will continue. Choice exerts from the Overall Synthesis include:
"approximately 60% (15 out of 24) of the ecosystem services examined during the Millennium Ecosystem Assessment are being degraded or used unsustainably, including fresh water, capture fisheries, air and water purification, and the regulation of regional and local climate, natural hazards, and pests"
"there is established but incomplete evidence that changes being made in ecosystems are increasing the likelihood of nonlinear changes in ecosystems (including accelerating, abrupt, and potentially irreversible changes) that have important consequences for human well-being. Examples of such changes include disease emergence, abrupt alterations in water quality, the creation of “dead zones” in coastal waters, the collapse of fisheries, and shifts in regional climate"
Just read the synthesis.
"the harmful effects of the degradation of ecosystem services (the persistent decrease in the capacity of an ecosystem to deliver services) are being borne disproportionately by the poor, are contributing to growing inequities and disparities across groups of people, and are sometimes the principal factor causing poverty and social conflict"