Using a graph DB with many underlying KV-store nodes, you can have a single graph spread over many machines representing e.g. Facebook's social graph, and run a query which "chases around" edges between vertices that live on different nodes, to solve that query, while ensuring that as little of that has to happen as possible — both by rebalancing vertices so that data is sharded at low-connection-degree points in the graph; and by consolidating the steps of queries that occur on the same node into single batch queries, such that the whole thing becomes (close to) a single map/reduce step.
There's nothing in Postgres that knows how to do that; if you had e.g. a graph stored in a Citus hypertable, and did a recursive CTE over it to do graph search, then you'd get pretty dang bad perf.
