After all, if it's compromised, you can't change it.
Naked Security explains:
People have been banging the drum for years, but perhaps now the massive Equifax breach will force the issue to the forefront: it’s way, way past time to dump social security numbers (SSNs) as a national ID in the United States, as SSNs are a terrible way to identify or authenticate yourself. Here’s why.
You can’t change them if they are compromised
This tweet by @SarahJamieLewis sums up the issue quite nicely: