It’s not for your personal privacy, or to spare you personal embarrassment. But rather because large-scale demographic data collection is dangerous.
The Nazis used such collections to locate Jews. America used such collections to locate Japanese-Americans. The Rwanda genocide was facilitated by tribal affiliation being printed on ID cards. In none of these cases were the data collected for the nefarious purposes it was eventually used for.
Information is a form of knowledge, knowledge is power, and power in the wrong hands is dangerous.