It's a good thing that relational databases solved the problem of generating efficient execution plans for ad-hoc queries about thirty years ago—with SQL.
The point is that relational databases will serve as the data model for most GraphQL application servers. Obviously, end-users don't have to write their own queries.
Also, keep in mind that most big websites aren't just a single box you can query. A plan for fetching data across >>1000s of boxes works a bit differently than within MySQL or whatever.
Usually you end up making layers of abstraction / cacheing / parallelization atop them that don't have an easy SQL interface for the frontend to query. FQL was heavily sanitized and abstracted anyway, not directly SQL onto the boxes--imagine that security shitshow!