Nothing economically prevents anyone from having children other than being unable to afford the food to grow one. But as one could imagine, very few people probably want children, if it also meant that they'd be homeless. In other words, someone's want for a house can be greater than their want for a child. And this can go for anything (from spouse, to life circumstances). Again, it's not that most people can't economically, it's more that many people don't want to sacrifice other things that they also want.