The whole plot is that Mother simply cannot return to the skies because there isn't a rocket powerful enough to launch the entire ruins (her body). However, to fight Mother, you must give her soul a physical form. Why didn't any of the children send her to the skies after giving her soul a physical form?
Because even then someone would probably need her power to do her job after she left, and who can be trusted with power like hers?
It's also likely that her soul can't be taken too far away from her body like that, and the effect of the Medicine of Life probably doesn't last very long in any case.
What is the true fate of the 4th Children? Mulbruk said they "they lost their purpose.", but what really happened?
What is the source of cold in the Graveyard of the Giants?