> Imagine if they demanded your SSN to sign up? A phone number is no different or less sensitive a unique identifier, perhaps even moreso these days.
The goal is for the service to have a unique identifier, and phone numbers happen to be a really good one to prevent spam also since it outsources verification of human entity to the phone companies.
> since it outsources verification of human entity to the phone companies.
That's not the reason phone numbers are used. They are used, because they are something you have in addition to something you know like an SSN or password. This is two factor authentication.
The US has plenty of centralized identity systems, including the Real ID one, a backdoor federal ID system that is required to board all commercial flights in that country.
The goal is for the service to have a unique identifier, and phone numbers happen to be a really good one to prevent spam also since it outsources verification of human entity to the phone companies.