More that young(er) folks could afford to live on a single income for the pre-school years. Or, I guess, that there's extensive parental leave and support for the parent doing primary caregiver.
There are many solutions to different aspects of the problem - if we define the problem something like "people get together older, and have kids older, and have fewer."
But even if everything was "easy and perfect" (arguably some other countries have this) - you still have something that is generally discouraging people from having kids.
The median Amish family income is about $65,000 and typically has six to eight children.
Mixed in with all this, and possibly preceeding all this, is declining marriage rates. It's significantly riskier, financially and relationally, to have kids without getting married.