Bad idea for a variety of reasons.
The first reason has been touched on above. Biometric scanners, be they fingerprint scanners, face scanners, eye/iris/retinal scanners, etc., do not produce the exact same image each time. They compare what they scan to the initial scan when you first set it up, and if similar enough, will unlock. "Similar enough" is not good enough for a private key. Even the slightest difference results in a different wallet and lost coins. Even if you managed to overcome this, then the second issue is that biometrics are not secure. They are not secure enough to protect a phone or a laptop; they are definitely not secure enough to protect your private key. You leave your fingerprints on everything that you touch, and they can be lifted and cloned without much difficulty. Face and eye scanners can be fooled with pictures, or with 3D prints made from pictures.
So the end result is either a private key it is near impossible to recover, or a private key which can be easily attacked.
I agree with what you have said: using fingerprints or other biometric traits is, at best, a highly insecure approach to generating private keys, let alone the fact that biometric scanners differ significantly in functionality and reliability, which may result in different interpretations of the same biometric data and thus loss of funds, or may cause mental or emotional disorders in cryptocurrency users relying on such devices and questionable cryptographic schemes. But the authors of the following paper
Two-factor-based RSA key generation from fingerprint biometrics and password for secure communication claim they have found a solution to the problems you outlined. Although it focuses on RSA, the same principles can be applied to ECDSA, I will quote from "Abstract" section here:
Abstract
In an asymmetric-key cryptosystem, the secure storage of private keys is a challenging task. This paper proposes a novel approach for generating the same public and private key pair on a need basis. Hence, the need for secure storage of the private key is done away with. The proposed approach for generating the key pair is based on two factors: fingerprint biometrics and password. A stable binary string is generated from the distances among pairs of minutiae points in a fingerprint using a gray code-based method. Experiments show that gray code representation significantly reduces the number of inconsistencies between the generated bit strings from two instances of the same fingerprint as against the binary code representation. Hence, the Reed–Solomon error correction code successfully corrects errors due to variations in multiple instances of the same fingerprint to induce stability in the generated string. Hash of the stable string generated from the fingerprint and the string generated from hashed password are XORed to derive a stable seed value. The proposed approach uses this seed value to generate two large prime numbers. These prime numbers are used to generate the public and private key pair using the RSA key generation method. This seed value ensures the generation of the same key pair every time. The experimental results show that the proposed approach can ensure a stable generation of the key. It is not required to store either the fingerprint template or the password. Moreover, the generated private key is also not stored. It can be regenerated on a need basis.