You need to understand the principles of
Malthusianism that the left are trying to achieve. The Malthusian left's ultimate objective is mass genocide. They believe that the planet can only sustain 3 billion people maximum, so their goal has always been to seek ways to limit the human population to that number. The left made a good start last century, wiping out more than 150 million people in the two world wars that they started, but that won't help them to wipe out 60% of humanity today.
So they have to be creative and somehow convince everyone that we don't need the resources we are consuming, that we are somehow better off diseased, starving, and living in caves (without any fire for warmth of course).
This is what makes leftists mentally-diseased - their innate self-loathing of their own species.