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.

Another reason: People might not necessarily know what they don't know right off the top of their head. So if you ask them something and they don't know the answer, they won't even necessarily know whether they don't know the answer because it is an embarrassing gap in their knowledge or if the question is hard. The only thing they might know for sure is that they don't know the answer.

Honestly, trying to bullshit your way towards an answer isn't a bad strategy in this circumstance.

Roughly speaking, what do you think your honest-answer/bluff ratio is?

Also, and I understand completely if you don't want to share your magic toolbox of secret interview questions, would you mind sharing an example of the sort of question you'd use here?