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 ans

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.