Why isn't there a specific code for "born in the US" for the CITIZEN variable when it's an optional answer?

The questionnare text page shows that the variable CITIZEN has an optional answer of “Yes, born in the United States.” Why does that answer code under N/A instead of having its own code?

“Yes, born in the United States” is not an available code because the CITIZEN variable uses categories that have been harmonized over time and samples. In other words, “Yes, born in the United States” is an available answer choice for some years/samples, but not others.

If you would like to see which respondents answered “Yes, born in the United States”, you can use the source variables available for your years of interest.