It is more difficult to control a massive population than a small number of people. The only reason anyone might give for population reduction is political control, fear of loosing power. Our technology is intentionally stifled, if we unleashed all science we currently have we could bring back extinct species of animals, grow food to feed every person on earth, supply every house with power and cure every disease for free! But instead a few assholes need to keep their control, money and reputation, so we cant do any of those things.