All a digital signature can prove is possession of a secret, it can't prove that some process was followed in generating an image.
It's impractical to have authenticity enforced by millions of consumer devices. If any random manufacturer in a third-world country can generate millions of valid signing keys to be embedded in millions of cheap phones or cameras, then there are many employees there that can also leak a bunch of valid keys if the price is right, and these keys can sign anything in a way that's indistinguishable from all these cheap cameras. And if you revoke the signatures of any "untrustworthy" manufacturers (really, any manufacturer can be and will be compromised, especially if some governments want to manufacture propaganda), then people won't stop using the phones/cameras just because their signatures aren't considered kosher, you'll still have millions of people uploading genuine, valid images from these cameras, so either people will have to trust invalid signatures or refuse lots of benign content, and I'll bet most people will choose the former one.
And of course a camera doesn't know if it's taking a picture of reality or another picture - it would take a relatively simple optical setup to allow any camera to record (and sign) an image off of another screen, so if some authenticity-verification system was actually working and popular, any reasonable criminals would do it and have signed recordings of their deepfakes from a real major brand camera.
It's impractical to have authenticity enforced by millions of consumer devices. If any random manufacturer in a third-world country can generate millions of valid signing keys to be embedded in millions of cheap phones or cameras, then there are many employees there that can also leak a bunch of valid keys if the price is right, and these keys can sign anything in a way that's indistinguishable from all these cheap cameras. And if you revoke the signatures of any "untrustworthy" manufacturers (really, any manufacturer can be and will be compromised, especially if some governments want to manufacture propaganda), then people won't stop using the phones/cameras just because their signatures aren't considered kosher, you'll still have millions of people uploading genuine, valid images from these cameras, so either people will have to trust invalid signatures or refuse lots of benign content, and I'll bet most people will choose the former one.
And of course a camera doesn't know if it's taking a picture of reality or another picture - it would take a relatively simple optical setup to allow any camera to record (and sign) an image off of another screen, so if some authenticity-verification system was actually working and popular, any reasonable criminals would do it and have signed recordings of their deepfakes from a real major brand camera.