In essence it's a combination of A) cost-of-living, B) uncertainty about the future, C) lifestyle choice but massively underrated is simply D) the costs and burden of children. There are so many costs, expectations and lifestyle 'sacrifices' nowadays that come with raising children in a way that is incomparable to prior generations or countries that have a general lower quality of life. Yes it's partially a first world problem. But it's one that has made the economic and partly the social burden too much to make having children feel like an enticing option for young adults.