Because "educate yourself" is quite possibly the most annoying, low effort thing you can say to someone whom you think should know something that they don't. Either share the information with them or don't engage at all; there's no need to be a dick about it.
In the case of North Korea, my excuse is "I haven't put the time into it because it's a small country on the other end of the earth -- nuclear armed, but without effective delivery devices and massively outgunned. I wasn't prepared to give a lot more detail in the context of an interview that isn't about geopolitics. If you want me to research I can."
