For sure you can create a REST Resource to retrieve all data you need, but what if you need to fetch only some fields, and this differ from a request to another? you will have to perform manual filtering, ... in other words you will reimplement what GraphQL aims to already provide (in a more abstract and clean manner), and this is just a small part of GraphQL.

Also, your observations look wrong. GraphQL is protocol agnostic. You can use an HTTP client to communicate with a GraphQL server. HTTP is even the recommended protocol.