Well, they don't.
Sleeping Beauty, in Perrault's, has serious mother-in-law problems -- as in, her mother in law tries to eat her two children and Sleeping Beauty herself. The Girl Without Hands Is thrown out of the kingdom with her child. In "The Wonderful Birch", the heroine is turned to a reindeer by her stepmother after she gives birth; in "Brother and Sister", she is murdered by her stepmother after she gives birth.
And then I was picking out patterns -- it's a heroine. And either she's not of royal blood or she can't return to her home kingdom. And the king leaves, so that her mother in law can send a false message about her giving birth, or her stepmother can slither in. . . .and all women?
Oh, no, then I remember Marya Morvena. Where Ivan is living in her castle as the heroines above are living in their husbands'. And then there are (some of )the frog princess ones, and the swan maiden ones where she isn't lost entirely. Where neither the man nor the woman is of royal blood. So variety there. . . .