If the biometric scanner stops you from shooting a target that someone else other than you determined?

They exist. For example, https://smartgun.com/technology and digging into the specs ... https://smartgun.com/tech-specs

    Facial recognition: 3D Infrared
    Fingerprint: Capacitive

I had a fingerprint scanner on an old phone and it would fail if there was a tiny amount of dirt or liquid on my finger or on the scanner. It's not big deal to have it fail on a phone, it's just a few seconds of inconvenience putting in a passcode instead. On a firearm, that's a critical safety defect. When it comes to safe storage, there are plenty of better options like a safe, a cable/trigger lock, or for carrying, a retention holster (standard for law enforcement).

You’ve never had your fingerprint scanner fail because your hand is dirty?

Or you didn’t press it in the right spot?

Or the battery was dead?

If I’m out in the wild and in a situation where a bear is coming my way (actual situation that requires carrying a gun in certain locations) I do not want a fingerprint scanner deciding if I can or cannot use the gun. This is the kind of idea that only makes sense to people who have no familiarity with the real world use cases.

I will admit i am arm chair designing something i have not considered deeply but all your edge cases sound like solvable problems to me and if they aren’t just not a situation to use this particular solution for. E.g biometrics on phones, open the phone for a configurable set amount of time and have fallbacks when biometrics fail (the pin) and emergency overrides for critical functionality like 911 calls. I was not proposing this be rolled out to all guns tomorrow by law but i am equally skeptical it is an intractable problem given some real world design thinking. Or constrain the problem to home defense weapons but not rugged backcountry hunting.