In Ly'Vuusia, there has been several incidents where princes or princesses were lose. The reasons vary, they could die, they could defect, or perhaps they could be stolen away. It's why we will adopt some, to make up for this fact.
And sometimes, those evil queens and witches we have to fight...are also princesses that have been taken from their homes. Never knowing what they want to do, they follow what they only know, never realizing the truth.