I think there is a lot of historical evidence that dismisses your claims. Polyamory, and/or communal parenting, has existed in many forms amongst many different indigenous peoples, and it is still practiced today. There was a time in our past where children being raised by many different parents was the 'norm', and an argument could be made that it is a more natural form of child rearing than our 'norm' of monogamous parent couples.

There is no evidence of people that practice communal or group parenting having issues with violence or jealousy, that is just your assumption. There is evidence that these kind of situations could be advantageous. The child has more people to pay attention to them and can feel a better sense of community. They are also being socialized better and are being shown a wider variety of perspectives, etc.

I would definitely suggest you look into it for yourself, if you are curious why your assumption is wrong.