My guess is likely because it seems like this would be impossible to implement without adding DRM to the smartphone and/or locking down Open Source image editors out of the attestation process. You would need to prevent access to the software, firmware, etc... otherwise the device could be virtualized or the program recompiled to circumvent the signature.
And for obvious reasons there's going to be pushback to adding that kind of DRM to smartphones. The tech does likely exist; this sounds to me like just normal attestation? It would likely hook into something like the Play Integrity API. A lot of people already hate the Play Integrity API though.
It's not the tech that's the problem, it is as you say, that people are hesitant to do it because it would require locking down the phone's software stack in a way that is widely understood by many developers and user advocates to be anti-user and in contrast to user rights to control their own devices and load their own software and/or firmware onto their devices.
I could maybe see an argument introducing some kind of signature to a raw camera input in firmware before it ever reached the user at all -- mostly just because devs seem to have given up the fight about custom firmware on a phone in general. But if you're talking about the phone signing the image after light editing like a crop has happened, at that point you're talking about moving this signature into user-space code, and while I'm sure that problem could have been explained better to you by the devs, it's not surprising to me at all that you'd get a hostile response to that suggestion because I don't see how it would be possible to do that without locking down user-space code.
