People don't talk about world population because population growth is mainly happening in the developing world where environmental damage is small per capita compared to the developed world, particularly the U.S. The environmental benefits of not having kids in the developed world are however commonly discussed, for example https://www.theguardian.com/environment/2017/jul/12/want-to-....

There's also a long history of people using environmental concerns related to overpopulation as cover for racism or other bigotry, so bringing up world population without qualification can feel, unfairly or not, like a racist dog whistle people are reluctant to engage with.