This has happened to every single society [*] as it industrializes [0], and offering extensive support and incentives to parents (e.g. as has been tried in Scandinavian countries) does very little to reverse this trend [1, 2]. My hypothesis is that as societies industrialize, they afford their population more and more activities that are simply more fun and rewarding than having children. So many people I know put of…
But this is by and large not true. I've traveled, eaten at expensive restaurants, enjoyed a child free existence into my mid thirties and having kids is a blast[*]
My wife was adamantly against having kids when we got together in our twenties, and she changed her mind in her late thirties; she now says that was the best thing that has happened to her.
Finally, there are many studies showing that people lead happier lives when they focus on someone else, have a higher purpose beyond just hedonism or living selfishly.
[*] - I have a supportive partner who collaborates with me so we both get time away and time off from the occasional drudgery or exhaustion of parenting. I still get time to work out, see friends, have time with my wife, all of that. If I couldn't afford the occasional babysitter and had a partner who was absent most of the time, it would be a lot harder!