You might be misunderstanding some of the dynamics here - it is really hard to successfully conceive a child if you are currently starving, because a very low BMI will interrupt a woman's menstrual cycle. However, a lot of people live extremely marginal lives - for example one failed harvest away from going into calorie deficit, or reliant on transitional forms of employment. So people might conceive a child in relatively good circumstances, and then find that they hit an economically challenging period before the child is five. Children are very susceptible to starvation that an adult can survive too, unfortunately - so even quite a small income shock could be lethal for a small enough neonate

Primary factor in my opinion:

People like children and family life, most people will want to have a family. the viewpoint that it is/could be morally wrong to have children is very uncommon outside of some western and internet based communities, in fact Im fairly confident the opposite (that having children is morally good) is more common

most people consider their lives worth living, therefore would presumably consider their childrens lives worth living

A few other factors:

in cultures with the highest birthrates, starting a family is often considered the default, and it would be a specific and unusual choice to not do that

Education for women, wealth, and access to birth control are all negatively correlated with birth rates

You mentioned some of these, but I think the strongest take-away here is that you are assuming most people share your viewpoint on several very specific and not-universal moral codes which suggest that people should not only not have children under certain conditions (eg poverty) but also go to great lengths to avoid pregnancy