people's faces change with some regularity. Also there are people who are incapable of recognizing faces, or just especially bad at it, so the method is unfair to them.
As for the issue of prosopagnosia, that's interesting, I hadn't thought of that. I think that faces could be used just as a bootstrap, equivalent to a "forgot my password" process, to enable someone to associate a blockchain-facial identity to a public key generated on a device. (Also, apparently people with one type of the condition, associative prosopagnosia can have the ability to answer "same/different" about two pictures, even if they can't tell you who is in the pictures.)
I haven't seen a full threat model or a fleshed out protocol of how all the pieces in the system would interact, but it's good to hear some of the hurdles it would need to overcome, thanks.