I simply don't want to deal with spam or scams. If I'm exposing my contact details it would be a separate set that is dedicated to dealing with communication coming from the public.
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.
Alright. Where I live, phone numbers are public info and not secret so here they are not used when authenticating yourself against a bank or similar.
Using a phone _number_ to authenticate yourself against the government seems completely bizarre to me.
I understand that leakage of phone numbers become a problem if you use them for those kinds of things. But honestly, using them for authentication seems crazy from a security point of view.