> The main problem GraphQL tries to solve is overfetching.

GraphQL is solving another problem. Problem of communication between frontend and backend team. When frontend team needs to have yet another, field exposed it needs to communicate this to the backend team. GraphQL let's them do this with code instead of Jira ticket and now the communication between the teams can be done asynchronously and batched. No more waiting for backend implementation each time. And if backend exposes too much then it's a backend problem and the frontend has nothing to do with it so it again can be solved without granular communication between backend and frontend teams.