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Your last sentence highlights the problem. I can have a bot that posts for me. Also, if an authority is in charge of issuing the certificates then they have an incentive to create some fake ones.
Bots are vastly more useful as the ratio of bots to humans drops.
Also the problem of relying on a nation state to allow these certificates to be issued in the first place. A repressive regime could simply refuse to give its citizens a certificate, which would effectively block them from access to a platform that required them.