* people can afford fewer children, and many no children at all
* I'm guessing here but possibly people are coupling at a lower rate, and are living single long-term
* more people happy to say that they just plain don't want children, and have other interests instead
People are much more able to afford children today than in the past, and some of the most affluent places (e.g. DC) are the most childless.
Cost is the biggest one I can think of. If you didn't walk out of the hospital with a 8k bill (on a good delivery) and then have to figure out how to afford $1,500 per month childcare after 12 weeks unpaid leave, people would probably have a lot more kids.
https://data.oecd.org/pop/fertility-rates.htm#:~:text=Fertil....
If people can't have children you can't fix that as easily as the factors causing people to not choose to have children.
A change to tax codes can immediately help alleviate the latter, for example.
Of course you are right that the underlying reasons are all relevant depending on the question, but they are all covered under the umbrella of "fertility rate".
In conclusion I am (pedantically?) arguing the definition of the term Fertility Rate, not anything else.