It wouldn't be a good idea, because she'd lose her life in the human world, or because her Ganma family wouldn't be the kind of family she's used to or needs? All of the above?

Do either Chiyo or Riko have jobs and/or go to school? Riko seems quite willing to pack up her life and flee with her friend, although it does seem like she's known the family for a long time and is putting on a brave face for Chiyo. Maybe she's not particularly settled in life, either. Or maybe she's just really singularly devoted and is full of Youthful Passion or something.