Because unlike a marriage, buying a house, or moving to the big city, having kids is a colossal life decision, with life-changing consequences that you can't reverse.
You can divorce, sell your house, file for bankruptcy, or move back to your parents' basement, but you can't unhave children after having them. You can't go on a break from your children, even to take care of your own physical, mental, or economic health.
Most people at 25 don't have any of that shit figured out yet, and in the modern day and age, they don't have the support network of grandparents, that prior generations had.