I think the 'Graph' in GraphQL confuses people. GraphQL is no more relevant to a graph database than it is to a relational database. It is an intermediary that facilitates efficient communication by standardizing req/res structure.

So, for people more comfortable with SQL, your question could just as well be "what can SQL do that GraphQL can't"?

The answer is, of course, that they occupy different domains and have different functions. Both can do lots of things that the other can't.