I think that a lot of people writing off having kids are underestimating their potential as parents, but that's their own beef to hash out.
It's worth noting a few other things:
A) the West is not experiencing a Golden Age of prosperity right now, unlike in a good chunk of the 20th century. College education does not mean a steady career where one parent can support a whole family in a comfortable middle class existence. Both parents being required to work puts a strain on time and energy for children.
B) We have experienced a radical change in societal values among younger folks. Church used to be a cornerstone of the community, and was where a lot of social interaction happened, and church strongly encouraged the nuclear family. We've turned more inward, making communities online and at a distance, and I think that does have an effect on people's traditional views of social ties. There's less unconscious peer pressure for conformity, where thinking that having kids is just the thing you do, if you're not seeing other people's kids around or hearing them talked about.
C) Lot of controversy about this in certain circles, but facts are facts, women today are free to pursue their own goals to a much greater degree than in the past. It started with suffrage, got rolling with staffing factories in WW2, and by the turn of the millennium females having careers was the norm, not the exception. Women do not choose work just because they have to for economic reasons, but because they want to. And the whole pregnancy and child rearing thing gets in the way of that.
D) I think millennials (of which I am one) and later generations have been raised on what I call a "Disney media diet", but it's by far not restricted to just Disney. Basically, a lot of children's media for a while now have had themes of empowerment in general, individualist values, following your dreams and valuing experience over material. I think this has contributed to a general feeling of wishing to pursue one's own interests. Now, I don't have kids, but I've spoken to people with kids and people who haven't. People who haven't say that they think kids would force them to change how they spend their time and energy, and what they spend it on, in a resentful manner. You can't veg out to a videogame for 10 hours on a Saturday when you want, when their's a kid in the house. People who have had kids say, yes, that's true, but that they don't resent the change, because what they valued spending time on changed when they have kids. And I think raising a kid, while filled with its frustrations, would also be a lot more fulfilling than the veg sessions of games and TV you miss out on. I think a lot of folks who write off having kids discount this too much. But it's also true that having a kid is about the biggest responsibility you can have, and if you're not willing to embrace that, you're doing yourself a favor by swearing it off.
Edit: saying "we need to get people to have kids to perpetuate our culture over them foreigners coming over here and usurping it" is hella racist and, quite frankly, is talk the Nazis did back in the day. Godwin's law invoked. It's just gross.