Why do developed countries have a smaller population density than developing countries?

1) the term 'developing' not only entails that its economically developing, but also socially and perhaps even politically.

2) talking about population density; most developing nations have access to basic medical requirements, and hence mortality rate has gone down. That's reason number one.

3)Talking about society, unlike developed societies, buying a condom or contraceptives is still seen as a taboo on a surface level, thereby reducing the reach of such measures, countering population control.

There can be several more, like lack of awareness about family planning and there are certain nations where each child is seen as a means of better living (through child labour) and hence favours the opposite of population control.