Well, the mountaineer reason of ”because it’s there”, which honestly is probably the more honest and moral of the two. To become a ”mulit-planet species” in a real sense we need more earthlike planets, else the iss in orbit around the moon would be enought to make the claim. Consider that we don’t cout ourselves as an underwater species just by virtue of submarines. Even if we made a self sustaining underwater base and condemneded people to live there forever it would just be those poor people living underwater, we aren’t suddenly a underwater species.