But different families have different levels of income, and there is a very long tail. If nannying paid better, it would just be a smaller fraction of families

But different families have different levels of income, and there is a very long tail. If nannying paid better, it would just be a smaller fraction of families that could afford it. The fact is because it's a low skill/high supply labor pool, the equilibrium point is reasonably low, such that even many middle class households can afford nannies. For software yeah your cost could be borne by a billion people, but those billion people could instead bear the cost of a cheaper developer if they were available.