Also, it could just as easily be faked, like so much else. Even if someone offered such an assurance, I wouldn't trust random strangers on the internet. At best it would become an arms race between "this is not a real person" and those who sell fake assurances. And it would be one more weapon to damage anyone's reputation you don't like. "not a real person!"
By default you could choose not to trust. If you don't trust random strangers it's OK. So I don't see how it would damage someone's reputation worse than today.
It would potentially only add trust. Arguably, today we have bots because we have no other option than to trust without justification. With the public social graph, the trust above the baseline would be justifiable.
This model was used in the beginning of the web, where links to your site from a well trusted website would increase the score and therefore SEO of your own site. This very quickly turned into a business of people selling hidden paid site links, and evolved into the paid editorial model we see on every news site today.