The reason is because the state has an obligation to take reasonable efforts to ensure the basic human rights of all people within its jurisdiction are protecte

The reason is because the state has an obligation to take reasonable efforts to ensure the basic human rights of all people within its jurisdiction are protected. That is, the state has a duty towards the children that is independent of the state's relationship with the parents. There also is a fundamental reason to trust the state over a child's parents: the state has considerably more transparency and oversight over its actions than the parents do.