Because they want to and/or it’s socially advantageous! Inheritances work the same way they do in some countries in our world where same-sex marriage and families are legal. A gamete donor or surrogate isn’t considered an additional parent legally (in some cases people would co parent but that’s not covered in the law - this varies in non-Westerlin countries); the parent couple get put on the birth certificate and pass down their inheritance to the child unless there’s some kind of drama or legal complication that means they don’t.
Culture and social norms reinforced over thousands of years in Westerlin mean that there isn’t the worry about “who someone’s biological father is” that there are in a lot of cultures in our world. The important thing is that people A and B are recorded as parents of child C. Which will have its own challenges, but not related to which genetics child C has or who exactly gave birth to them.