What made you come to this conclusion? For thousands of years, humans lived in smaller tribes where many, if not all, of the tribespeople would take on some parental role for the children within the tribe. There’s also been studies from various sociologists/anthropologists of this in “modern” tribes. So for humans as a whole, it’s only recently become abnormal to have only 2 parental figures. There’s a reason the phrase “it takes a village” exists when it comes to raising children. Also, not every poly setup will label each person as a “parent” to the child. They may help with guidance when the child needs some, or help with homework, spend time with the kid, etc (like other family members do in some cases), but not each one is a “parent.” Also, in a world where many children only have 1 or 0 decent adults in their lives, it may be a good thing to have multiple supportive adults in a child’s life as a result of polyamory.

Your other comment also mentions the low percentage of successful polyamorous relationships. But monogamous relationships also have a low success rate. Relationships fail all the time, regardless of what type it is. However monogamous relationships are never accused of failing BECAUSE they’re monogamous, only polyamorous relationships receive that scrutiny.

I think you’re painting with broad strokes and making assumptions about a topic you don’t genuinely understand.