As the sibling post noted, most of the positive natality in Western Europe is due to immigrant mothers. And since the US has quite a strong barrier to entry for immigration they don't benefit from this effect.
[edit] Looks like in term of raw numbers, the US has much more immigrants than Germany for example. But in terms of legal immigrants there's about twice the number in Germany vs. the US for 2021 (dunno how reliable statista is though).