[throw2500]

Because the world is impure and the link between the real world and the mathematical domain will never be ironclad.

For instance, say that Joe makes a deepfake and then signs it with his key. Sure, it's beyond doubt (assuming keys weren't leaked, etc) that Joe either took or created the picture, but that doesn't in itself tell you whether Joe made a deepfake or not.

It's the same as in supposed blockchain logistics operations. If Fred the farmer says he harvested x bags worth of grain but some were stolen before he could ship them, there's no way to mathematically verify whether the theft actually took place or the harvest just came up short.

In both cases you're going to need some kind of monitoring, and that's the purpose deepfake detection algorithms would serve.