This is core to the Gricean Maxim of Quantity [1], according to which one gives as much information as needed but no more. If the sign says "No vehicles in the park" and nothing else then any reasonable person should assume that the reason for the sign is so obvious that no further clarification is needed.

Unrelated, it is also the reason why a hot-dog is not a sandwich, pragmatically speaking.

[1] https://www.sas.upenn.edu/~haroldfs/dravling/grice.html