My wife's mother beat and choked her from childhood. She was regularly told she was worthless and unwanted. Her father abandoned them, and her stepfather would whip her with clothes hangers for imagined infractions.
Many people are in similar situations. The abuse may not always be physical or as extreme as what she endured, but many parents are unfit for parenthood, and their children suffer from the trauma or neglect or religious extremism for the rest of their lives to a greater or lesser degree.
We don't always hear these stories because our social bubbles may comprise people who are more well adjusted, or at least present so. But abuse, infidelity, narcissism and so on are not new, and they aren't as rare as we might feel. And if those kinds of people didn't reproduce, that would be a good thing.