So far, we trust GitHub that a certain repo is under control of a certain individual or organization.
We trust Twitter that a tweet was done by Elon Musk and not by Joe Doe. We trust Instagram, that Sue really got a like from that famous influencer and did not fake it. We trust HN, that I wrote this comment.
But if signing these things would become the norm, commits, tweets, likes and posts would become eternal. Independent of any central authorities.
Unfortunately, it will probably not become popular. As there is nothing to gain for the author of a commit by signing it.