Identification is knowing which account to log in. dustinmoris, User123 or user124.

That alone is useless, as anyone who knows your name could log in. So we need to add security to authenticate you. To authenticate with reasonable certainty that the person accessing dustinmoris actually is Dustin Moris, at this moment willingly accessing their own account, willingly transferring £1m to NeedMoreTea. :)

Now, it's certainly true that biometrics are uniquely associated with you, but prove identity not authentication - bear with me here.

You leak DNA and fingerprints all over the place, constantly. Fingerprints have even been picked up from photos. That makes fingerprints surprisingly weak authentication. Face IDs have been fooled by video and photos. I'm not at all current on what the state of the art on FaceID hacking and devices is, so can't say more there. In a police interview they can place your thumb on the phone or wave phone under your face. Maybe borrow your thumb while you sleep to transfer that £1m. The bank will say you fully authenticated it, in their highly secure app, you will deny it.

In combining biometrics as identification and authentication we've compromised the system. It does not give certainty, or even especially high confidence. If motivated, anyone in your office could lift a print while you're out. Computer or phone says Dustin Moris is believed awake and accessing, but you're certainly not doing so willingly, nor can you resist if just a touch or look unlocks. In this context, your wife might easily pick up on whether you were willing in a police interview or not, the phone can't, because mere identification is the unlock.

With a password they have to persuade you to reveal it. It is very easy to not leak passwords accidentally on PostIts, and manage the complex with a password manager. The police can certainly beat it out of you, but most places are supposed to have rules against that. Many places actually honour those rules. A judge might question the bruises and missing teeth.

For a lot of people that's a fine distinction that doesn't matter, the convenience of it probably being you willingly unlocking the phone with your thumb is good enough. If security actually matters, it's not.

Sorry that got a bit long.

In practice the computer is just getting a picture, which is semi-public information, and that's why the only thing you can rely on is that it's an identifier. It's not enough to authenticate when you really need security, only in more casual situations.