I wonder how much of the U.S. data is driven by people with very little economic advancement to look forward to in the more economically-depressed southern states. A lady friend of mine who moved to Mississippi for a few years after high school (no idea why) told me that the most common question she got from strangers is how many kids she has, as though it is taken for granted that by the time you are 18-20 years old, you are going to be having children because what else is there to do? You get married (or don't) and start having kids, since it's not like there are tons of high paying tech jobs to compete for, since there isn't really high paying anything there for anyone. Meanwhile in the northeast/New England and the west coast, where there are not only those types of forward-looking jobs (i.e., not coal-mining) but also some of the finest advanced educational institutions in the country/world, there's a lot you can do if you don't just want to have kids in order to give your life meaning.
People can only live as good a life as their options allow, and there aren't a lot of great options in a lot of the United States, sadly.