Because biometrics are usually relatively publicly accessible information. Passwords aren't. You're arguing reproducibility. Well, your face can be replicated by a picture you put on Facebook, fingerprints are left everywhere you go.

Usernames are fixed values and are generally public. Biometrics are also fixed values and are generally only slightly less public. They're both identifiers.

Passwords can be changed and are secrets. They're authenticators.

The difference between them is exactly the difference between identifiers and authenticators. Misunderstanding this difference causes tons of issues, in a wide variety of situations. The most notable one recently is probably Social Security Numbers being used as both, which leads to identity theft.

Perhaps I was too flippant. Point being, the “public availability/replicability” of the biometric would seem correlated to the point on the username->password continuum.

This will probably matter less once our future devices can interact with our sci-fi personal nanites, or rfid implants in the meantime.