It's not. The fragments you can execute are limited if you do it right. A client isn't allowed to just execute anything it wants, because the valid operations are pre-determined. The client sends a reference which executes a specific pre-planned fragment of code.

In development, you let clients roam free, so you have access to the API in a full manner. Deployments then lock-down the API. If you just let a client execute anything it wants in production, you get into performance-trouble very easily once a given client decides to be adventurous.

GraphQL is an execution semantics. It's very close to a lambda calculus, but I don't think that was by design. I think that came about by accident. A client is really sending a small fragment of code to the server, which the server then executes. The closest thing you have is probably SQL queries: the client sends a query to the server, which the server then executes.

It's fundamental to the idea of GraphQL as well. You want to put power into the hands of the client, because that's what allows a top-down approach to UX design. If you always have to manipulate the server-side whenever a client wants to change call structure, you've lost.