Yeah, and many Americans might be happier living in different sorts of groups than a single-nuclear-family house — such as an extended family, a group of friends, or occasionally even a monastery or something.
What's more, it's probably healthier for children to grow up with a larger set of caring adults; and people might have more kids if there were more and better ways to spread the costs and work of raising children!
A person goes to college, works a few years, meets somebody and doesn’t want to rush into anything—maybe wants to pay off student loans first or save for a house, which takes a few years or more.
Maybe? Most people I know in this scenario wait until they have their financial lives in order (have enough money to save a little something after paying for all of their expenses including student loans) before even getting married. If they can't save they are typically not getting married. And then they try to save more before buying a place before considering having one child. By the time they reach a stable financial level they may have aged themselves out of a larger family.
The only people I know who are having kids in their 20s or earlier into their relationships are having unplanned pregnancies.
Maternal mortality rates (especially Black maternal mortality rates) are sky-high for the OECD, and a good chunk of the country has banned abortion. Maybe people who can bear children also want to minimize their chance of death.
Edit: Also? Climate change. People want a better future for their kids, not a smoldering hellscape.