I'm surprised no one's mentioned the climate factor, both in terms of actual climate and "living climate" (as in, the world we live in and the conditions we get to live in).

A concern I see typically come up when discussing having kids with friends is the strong belief that the world will certainly be significantly worse off for them, if not having water wars maybe even during our lifetime.

Most people agree they'd rather not have children than bring them into the world to live through the nine circles of hell.

Of course, it's also possible this never materializes, but the fact that it is in people's mind alone is enough.