> Because you have no right to kidnap someone else’s child. Humans also have a right to live. Parents with better opportunities(whether through a better lifestyle or through better medicine) will be able to grant a better life expectancy for the child than the biological parents. This means that while there's no defined right to "kidnap" someone's children, the non-existence does infringe on an existing right.
> Because the human rights of the parents to raise their child and the rights of the child to be raised by their parents both determine that you shouldn’t. Ditto.
> Because you have no authority to determine what is better for the child And who does exactly? The people bringing the child to this world without consent? The government? Hypothetical better parents? Once a child grows up to a sufficient age we assume that the person has gained enough sufficient mental capacity to decide for themselves(which is also debatable but let's leave that for another day). That line gets only more blurry the younger the child is and how do we know who exactly has this "authority" to determine this?
Modern family structure gave this authority to the biological parents due to the complexity of the question above but this is still a debatable point in my opinion. I heavily doubt that the status quo will be challenged in the near future.
> Because the ends don’t justify the means. I agree with this as taking children of people can cause significant distress for reasons I assume evolution related. And moving whether ends justify the means or not aside, no one has the authority know whether an outcome is the "better" outcome for a child or not so the ends are also blurry.