Because on the current and foreseeable techlevel it's physically impossible. You either have modern industry and the ability to feed 8 billion: then you need to burn fossil carbon, if not for electricity, then for transport -- or you run on solar and wind: then billions starve.
This might sound pessimistic, but I’ve always wondered why no billionaire has single-handedly funded the halting or reversal of climate change purely for the narcissistic thrill of being able to say they literally saved humanity.