That requires a central registry match cryptographic identities with physical ones. That is overhead and a trusted third party. An idea like this was also ofte

That requires a central registry match cryptographic identities with physical ones. That is overhead and a trusted third party. An idea like this was also often brought up in spam prevention, which interestingly is also the origin of proof-of-work: https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash