[felisml]

Because countless companies are stupid and treat it as authentication.

It's a password that you reuse everywhere and never change.

[eru]

By the way, you _can_ change your SSN. Nobody ever bothers to do so, though.

[nommm-nommm]

Only under certain circumstances.

"The SSA may assign a new Social Security numberto you if you are being harassed, abused, or are in grave danger when using the original number, or if you can prove that someone has stolen your number and is using it."