Your missing the history part. Obviously any signature in isolation is only as good as your knowlage of that person. Thats how cryptography has worked classically. A block chain is not just a hosting service. Of course that could be done cheaper. The point is to have imutable history of what information is provided. This allows a reputation system as well as the ability for parties to endorce or rebuke information in a way that can not be covered up later.
Trust requires reputation. No anonymous person in isolation can be trusted. The point is to allow collaborative verification with proof. And that can not be done by a centralized host or authority without giving that authority control over what is “true”.