Your RAG can't answer 'why' -- GraphRAG finds what vector search misses

The question that broke my RAG pipeline I had a solid RAG setup. Embeddings, vector, top-k retrieval, the whole thing. It handled factual lookups just fine: "What's the API rate limit?" "Which config file controls logging?" Quick, accurate, done. Then a teammate asked: "What technical challenges do Project A and Project B have in common?" The system returned chunks about Project A. Chunks about Project B. Individually relevant. But it never connected the dots between them. It couldn't, because vector search finds similar documents -- not related ones.