And that is exactly why they can be changed - because they're valuable details that can be used to track someone down. Your DNA is easily obtainable and is not used in any meaningful way that would affect your life if it was exposed.
For me, phone numbers have had reduced importance over the last couple of years. Most of my communication with other people are over various messaging apps.
Identifier as in, used for authentication and possibly even tied to your real name. Even messaging apps don't tend to have screen-names like AIM, they have phone numbers, including the so-called "privacy-focused" Signal. Tons of in-person services (govt, banks, etc) will also want one, and it may be used as yet another piece proving your real identity.
