Otherwise malicious actors will just strip the metadata out of the generated one.
How could that possibly work? In order to mean anything, it would have to be signed by some independent entity who can verify the photo is real. How would that entity be able to tell if it is? And what would be the ramifications of requiring everyone who takes pictures to get them signed?
Even if it were an effective solution, isn't it just pushing costs being created by AI onto uninvolved others?
The camera uses a certificate to sign the authenticity of the photo taken and some metadata. Any editing app wraps that signature with its own signature stating it was modified. You end up with a chain of provenance. And any generated photo is either missing a chain, or when un-wrapping, its origin cert isn't from a camera.
There are just too many ways this approach can be subverted.