Because people won't give up their modern lifestyle and no country will support the massive programs that would be necessary to reverse the warming trend.

Resource consumption is the root of all environmental impact. High populations tend to exacerbate that impact to the point where it is negative. So once the population reaches a certain level, even a minimal existence creates a negative impact.

Good luck convincing even a majority of the population to reduce offspring (capitalism as we know it requires population growth), reduce consumption (people are greedy and wasteful), and change their lifestyle (from eliminating chemical products, adopting a zero trash lifestyle, to giving up cars, yes even the electric ones). Then you'd have to get countries to work together not just to essentially ban greenhouse emissions, but also reverse the warming trend by blocking solar energy in space or the atmosphere and recapturing greenhouse gasses, etc.