Can't they just wish for those things instead of marrying into it?
Well, the system of becoming a Princess of Zahard is quite complex and is probably pretty far away from what you think.
I'll start you off with a quote that describes what it's like being a Princess of Zahard
And for this reason, they are frequently referred to thus: 'Shoes displayed in the King's showcase'. Although very beautiful, nobody can take them out to wear.
Princesses of Zahard or Daugthers of Zahard are a group of elite females chosen to serve Zahard, as the obligation for receiving his power. Considered as the "ultimate species" in the Tower.They are an existence which, at times, serves as a sword to defend the authority of the King and at other times, is worshipped by the citizens of the Tower".
Basically Princesses are female regulars who have been chosen to receive Zahards blood and power. These females are nominated and tested by 4 categories. Power, Resourcefulness and Appearance. But the most important factor evaluated is if the girl is capable of properly receiving his power.
The most outstanding children are then granted Zahard's power through a special method, further enhancing their combat and other skills. Hence, Princesses of Zahard are physically stronger than most members of the 10 Families; furthermore, Regular Princesses are more "durable" than an average Ranker.
Which is saying something considering the difference.
They also have to follow these rules or face death for treason.
1. They must not have a relationship with a man.
2. They must not marry.
3. They may not have a child.
4. They must not help an enemy of the Empire.
5. They may not enter the Floor of Death.
The first three rules are to prevent Zahard's power from being spread.
In layman's terms being a Princess of Zahard is to become a powerful, strong, and influential Regular that few in the tower can match. But it also means never being able to be truly free.