It's because they're a scam. Point the camera at a forged image with a higher resolution than the camera sensor and it will make a signed copy of the unsigned forgery.
That's before getting into the practical problems with securing the keys. Every camera by every manufacturer has keys in it and the attacker only needs one key from one camera, and they get to choose the model? Creating something premised on needing to trust something with such a high probability of being compromised is worse than nothing, because it allows the ensuing forgeries a mechanism to pass themselves off as "signed" "real" images.
That's before getting into the practical problems with securing the keys. Every camera by every manufacturer has keys in it and the attacker only needs one key from one camera, and they get to choose the model? Creating something premised on needing to trust something with such a high probability of being compromised is worse than nothing, because it allows the ensuing forgeries a mechanism to pass themselves off as "signed" "real" images.