It’s ideology. People living in religious communities often worship fertility, and try to make breeding as widespread as possible. Blood based nationalism is obsessed with the “race” (unscientific term) not dying out. For both, having children and raising them in the same ideology is crucial for preserving their existence.

For the global liberal capitalism, the only way it can maintain its egalitarian promise is for it to rely on infinite growth. You can promise to the exploited that they will have even more people to exploit in the future if they keep their head down and stick with the status quo for long enough. It’s starting to hit external limits given by the environment, and available space only recently.

Meanwhile, some societies are aging and doing fine. The economy can be fine tuned to prioritize basic needs, and sustainably supply the people regardless of their age. For example automation in agriculture means that we’ll need less and less people to produce food. UBI is just one tool that could be useful here.