But do many countries treat them as secrets? Where I live, my number is a unique identifier for me but it's not secret. Because, you know, sharing secrets isn't smart and leads to the recurring issues we see in the US.
I'm from the US and I remember about 20 years ago I registered for a Blockbluster card (a way to rent DVDs from Blockbuster) and the form required putting my full social security number on it. In the US it's supposed to be secret but lots of places want access to it. Blockbuster never got my SSN and they did let me sign up without providing it. It's crazy they would even ask.