If a person is not born, then they seem to be deprived of the possible happiness they may have experienced if they were born. However, the feeling of deprivation can only experienced by existing in the first place. Therefore, a non-existent person does not experience deprivation of happiness.

If a person is alive, then they desire to be happy. A non-existent person does not desire happiness. On the other hand, there is a possibility of being deprived of happiness and experiencing suffering if a person is alive.

So, the only ones losing out are those who are already alive and may experience happiness if another person is born. Existence does not benefit someone who is not alive. It can only benefit those who already exist. Thus, birthing a child is only beneficial to those alive. A child is birthed not for the sake of the child itself but for the sake of those who exist. If one were to think selflessly only for the child, then they would not have a child.