We are at the same time experiencing all the challenges and consequences of depleting the planet of resources, a warming climate and shortages everywhere. Why can less people in a natural way not be part of the solution?
In your solution you also wouldn't have to kill people off. They would die naturally due to a lack of healthcare.
As for the why: because historically that has always been a solution to lacking specialized talent or simply not having enough hands to perform a massive task. You don't want to pay domestic labor, so bring in foreign labor. This time it's a general problem.
Because a major superpower dying out doesn't really benefit anyone except potential conquerors.