This has never made sense to me. Why people think terra-forming Mars and moving 10Bn people there would be a better decision than just cleaning up the current p

This has never made sense to me. Why people think terra-forming Mars and moving 10Bn people there would be a better decision than just cleaning up the current planet we have? The amount of effort to rebuild our forests would be a blip next to what would be required to move to a new planet.