Fairy godmothers develop a very deep understanding about human nature, which makes the good ones kind and the bad ones powerful.
Inheriting a fairy godmother role seemed an easy job . . . After all, how difficult could it be to make sure that a servant girl doesn't marry a prince?
Quite hard, actually, even for the witches Granny Weatherwax, Nanny Ogg and Magrat Garlick. That's the problem with real life - it tends to get in the way of a good story, and a good story is hard to resist.
Servant girls have to marry the prince, whether they want to or not. You can't fight a Happy Ending, especially when it comes with glass slippers and a rival Fairy Godmother who has made Destiny an offer it can't refuse.