Because humanity staying on Earth with no support from off planet settlements will have no way of preventing millions or billions from dying from these that will happen: massive meteorite strikes, supervolcano putting Earth in an ice age for a few years, a Deccan Traps like magma field boiling for hundreds of thousands of years and causing massive swings in cooling & warming the entire time, or a gravitational disturbance changing the Earth's orbit even slightly will dramatically affect the climate, but could just pull the Earth from the Sun entirely and turn us into a rogue planet. Future gravitational disturbances that affect climate are certain; going to the extent of rogue planet status isn't.