Well, I guess you've never watched a Peter Zeihan video on youtube. Every third one relates to various demographic catastrophes happening around the world, but particularly in Russia and China.
Long story short, there's almost nowhere in the world where births and birth rates AREN'T declining. It's not a US problem, is a worldwide problem. Now there's plenty of reasons. As many posters here have pointed out, there's a very large trend with the young here in the US where they don't feel they can afford a family, what with the housing costs, the cost of living, and the cost of health insurance, unsteady job market, etc. This is also reflected dual income households...both parents need to work. This has existed for awhile.
Another relevant point is that...we're not an agrarian culture anymore. Families aren't having kids anymore because they don't need free labor. Then you've got the social ineptitude/disinterest things you're starting to see here in America, and which has exited in Japan for awhile. Kids don't want to fuck.
There's dozens of different reasons, and they're somewhat different in each country. But The last thing I read is that while America is in decline for births and birth rates, we're actually one of the countries declining the least rapidly compared to much of the rest of the world. Another interesting point is that, demographically, it also makes sense for America to increase immigration to combat this issue. Of course, we don't have any jobs or manufacturing to support that, but it's a thing to consider.
https://ourworldindata.org/fertility-rate