Why does a production-grade Kafka pipeline treat 'exactly the message was delivered' as insufficient on its own, without also verifying business-level correctness?

Delivery guarantees (at-least-once, exactly-once) are a Kafka-level correctness property about the messaging layer, not a guarantee that your business logic correctly interpreted and acted on that message — a message can be delivered exactly once yet still be processed with a bug, so monitoring and testing need to validate business outcomes, not just delivery semantics.