Ask HN: Wouldn't it make sense to use GPG as the activitypub user?

I was thinking about this while trying to use Mastadon. I don't get why I have to create an account on every Mastadon server just to read. The thing is that GPG is already a federated "user identification" system so why not use that to sign posts. And with Mastadon doing the ActivityPub thing I think GPG should spread to ActivityPub too. Think about it, this is what GPG was designed for.