Where have you been that you don't know that it only comes from centuries of law and culture?
It's still automatically how they are identified as on their birth certificate, and good luck with a census taker although now we have options. Cpl decades ago one chuckled and wrote me down as "Hispanic" after I told her "White," (which is also what it says on my birth certificate) because I had mentioned before that my mother was from PR, even after I told her, "No, I said White"