In many parts of the world, it is common for grandparents to take care of the babies and even small kids. In other words, free daycare. Not so common in the US.
As people delay parenting, this option becomes less available. When people routinely had kids in their 20s, starting age of grandparents was about 40-50, when they were still had a lot of energy to help. In a world where people have kids in their 30s, the average grandparent has their first grandchild between 60-70. It means a sizeable fraction of them may be already too frail or even already dead, to contribute significantly in childrearing, needing assistance themselves. That compounds the problem, because then people have to raise small children at the same time their elderly parents start to need more assistance.