Individualism has been the cultural focus of education and child rearing for a long time. Having children changes who you are and can stand in the way of achieving your personal or professional goals.
I'm not sure it's a bad thing but it's easy to see how that could lead to less people wanting to be parents, especially women who've had a much more dramatic cultural shift in the last several decades.
This is not even to mention that many parents simply do not care for their children well, as parental child abuse cases as well as the number of potential adoptees in the world is non-trivial. Now that is truly a cruelly selfish act more than any amount of not having kids can be, to bring a conscious entity into the world and then to let it suffer.
Or maybe parents are selfish? They ever asked their unborn child if they want to be born?
I’d also point out that the decision to have children is almost always one motivated by what people want for themselves, not some altruistic obligation. If asking people why they chose to have kids wasn’t taboo, I’d be willing to bet that the answers you’d hear back would be about what the parents want for themselves. At its core, “We’ve always dreamed of starting a family” is about the life the parents desire to live.
When it comes down to it, I think the real answer is selfishness. The sacrifice to their way of life, financially and otherwise, is too great for them. We can argue why there is a spike in selfishness - maybe it’s a reaction to an increasingly uncivil world, or maybe it’s due to social media’s hyperfocus on the self.
But either way, it’s more socially acceptable to say “raising children is prohibitively expensive these days” than to say “I can’t be bothered to change my solipsistic way of life.”