When talking to someone in person, if you say something, before they give a formal ack, their body language already tells you a lot about how they perceived the message.
If the reception was unfavorable, you have an opportunity to immediately add more to clarify if it was perceived incorrectly.
With a formal communication system such as an API, the API itself describes what messages it wants to receive and what the meaning of those messages are. But in human communication you’re not limiting yourself to messages the recipient has completely described. You’re creating new messages. New messages which the sender expects to be received ina certain way but there’s no guarantee it will be because we don’t know if the recipient is setup to handle it as you expect.