Because it is tied to population growth and productivity:

The optimal number of children is 2.1, for offspring to replace parents

[1] https://www.britannica.com/topic/fertility-rate

When there is no population growth, no inflation or economic growth:

[2] https://www.youtube.com/watch?v=lSJis2K6B8Q

You might think that worldwide and US has <2.1%, but technology amplifies productivity, so it is not a 1-1 ratio

[3] https://www.worldometers.info/world-population/#:~:text=Popu....

[4] https://www.macrotrends.net/countries/USA/united-states/popu...