Because it solves all sorts of other problems, like having a well-defined way to specify the schema of queries and results, and lots of tools built around that.
I would be surprised to see many (or any) GQL endpoints in systems with significant complexity and scale that allow completely arbitrary requests.
Shopify's GraphQL API limits you in complexity (essentially max number of fields returned), but it's basically arbitrary shapes.
OpenAPI does the same thing for http requests, with tooling around it.
With typed languages you can auto-generate OpenAPI schemas from your code.
Yep, OpenAPI is also a good choice nowadays. That’s typically used with the assumption you’ve chosen a supported subset of queries. With GQL you have to add that on top.