The flipside is that unless you're hired to be someone's teacher, nobody is obligated to explain things to anybody else. There are also "the journey is the destination" type questions where the answer doesn't matter as much as the process of figuring it out yourself. Like science experiments in school where the answer is already known and the point is to teach kids certain ways of thinking or how to use certain tools. So yeah, there are legitimate reasons to say that, but a lot of the time it is indeed a cop-out. And your example is absolutely one of those times.