Because validity doesn't depend on meaning. Take the classic example: "What is north of the North Pole?". This is a valid phrasing of a question, but is meaningless without extra context about spherical geometry. The trick question in reference is similar in that its intended meaning is contained entirely in the LLM output.
I was not replying to your remark, but rather, a later comment regarding the "validity" vs "sensibility". I don't see where I made any distinction concerning wanting to wash cars.
But now I suppose I'll engage your remark. The question is clearly a trick in any interpretive frame I can imagine. You are treating the prompt as a coherent reality which it isn't. The query is essentially a logical null-set. Any answer the AI provides is merely an attempt to bridge that void through hallucinated context and certainly has nothing to do with a genuine desire to wash your car.
Because to 99.9% people it’s obvious and fair to assume that person asking this question knows that you need a car to wash it. No one ever could ask this question not knowing this, so it implies some trick layer.