That key will get leaked. A key that has to go into every phone, even if done at the manufacturer and onto the TPM chip, will get out.

Also even if it doesn't get leaked directly, the security of TPM chips is not absolute. Secrets from them can theoretically be extracted given an attacker with sufficient means and motivation. Normally nothing that's on a typical TPM chip would warrant a project of that magnitude, but a widely used private key can change that equation.

Plus a TPM chip doesn't really have means to tell the phone isn't being lied to. You could swap out the actual phone camera hardware and sensors for a custom board that feeds the entire phone camera data of your choosing and it would be none-the-wiser.