I don’t know. Ask Abraham’s wife.
Ask her what: how much she hated his concubine and her son? I think the Bible is pretty clear on that, actually.
Ask Sarah why she didn't see that a little bit earlier: that she and Abraham were supposed to carry out God's plan God's way.
While you're asking, ask Hagar how their reproductive scheme worked out for her.
And while you're researching that, check out Ishmael.
Reproductive concubinage --- the Biblical equivalent of surrogacy --- turns out to be a very fraught story, an imbroglio that continues to this day.