I can think of at least two reasons why. The first is that the candidate doesn't realise they are hitting their intellectual limits. The second is that the ca

I can think of at least two reasons why. The first is that the candidate doesn't realise they are hitting their intellectual limits. The second is that the candidate does realise the question is difficult, but doesn't know if you know the question is difficult. As a result, the second type of candidate has to decide whether to enter into a debate with the interviewer (and risk losing the job opportunity if the interviewer turns out to be ignorant), or try to appease them with a best-guess answer. If the candidate is desperate for a job, they might choose the appeasement option. You have no way of knowing if your candidate falls into the first or second bucket because interviews involve power asymmetry. In my opinion, this type of questioning is counterproductive -- you're looking for honest people by being dishonest.