The asymmetry that drives everything
A wrong answer and a refusal are not equally bad. A refusal costs the user a few minutes. A confident, well-written, wrong answer about a filing requirement costs considerably more, and — this is the important part — it does not announce itself. The user finds out later, from someone else.
Once you accept that asymmetry, a lot of design decisions that look overly conservative become obviously correct.
Why prompts are not enough
You can ask a model to cite its sources and to decline when unsure. It will mostly comply, and the failures will be silent and unpredictable. Making the behaviour structural — retrieval gates generation, sources are carried as data rather than as text — turns a tendency into a property.