“The unborn” are a convenient group of people to advocate for. They never make demands of you; they are morally uncomplicated, unlike the incarcerated, addicted, or the chronically poor; they don’t resent your condescension or complain that you are not politically correct; unlike widows, they don’t ask you to question patriarchy; unlike orphans, they don’t need money, education, or childcare; unlike aliens, they don’t bring all that racial, cultural, and religious baggage that you dislike; they allow you to feel good about yourself without any work at creating or maintaining relationships; and when they are born, you can forget about them, because they cease to be unborn.
Children aren't quite as good as the unborn, but they're close. Advocating for children still lets you feel good about yourself without having to actually associate with children. They're a group it's pretty much OK to be paternalistic toward. If they do resent your condescension, you can easily write it off because they're just children.