They've objectified a person. It's not a person they know and formed a relationship with. They wanted a "child" object "for" their lives (for themselves) so bad they spent money, went to a doctor, probably cried about it. They call it "unfair" that they didn't get the standard-issue fairy tale everyone is supposed to get. They feel cheated, as if they were entitled to it. The "child" is not a person to them, it's an object. It's a fungible token. They could have ended up with a different person and they wouldn't care; the child role is filled and so the fairy tale is complete.

I can tell you personally what happens when this person (the one they call "child") decides to go their own way in life contrary to the wishes of the owner/parent. Ostracism, coercion, blackmail, abuse, manipulation. Such disappointment that the person called "child" didn't follow the predetermined procedure, the fairy tale. It didn't do what it was programmed to do and they feel cheated.