Because if the USA reduced emissions by 50% and China increased theirs by 50% we would still have over double the emissions per capita the Chinese do.Indeed, so why would China be paid to continue progressing its economy on the back of greenhouse gases at the same rate it was before the Paris accord, and America(and other western nations) pay them while we reduce our economic growth by pursuing more ecologically sound avenues?
Developing countries like China and India have no reason to stifle the living standards of their people so we can keep wasting resources the way we do now.
Tom