Keyword search finds a document, but a person needs an answer. An employee asks "how tight should the flange be", while the procedure says "tightening torque for flanged joint fasteners" — not a single word matches, and the search returns nothing or a hundred irrelevant pages.
The second problem is more serious: much of a company's knowledge isn't stored as text at all. It lives in drawings, wiring diagrams, interface screenshots and tables embedded as images. For full-text search, such pages are empty.
And the third, which makes simple solutions fall apart on real documents: you can't know in advance how much has to be read to answer. One question needs a paragraph, another needs a table twenty pages later, a third needs a look at a drawing. A fixed workflow can't cover that.