Fertility rate is defined as number of children per woman [1]. Wether this is because of human choice or something else is irrelevant.
https://data.oecd.org/pop/fertility-rates.htm#:~:text=Fertil....
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.