Excessive willful/unwillful ignorance is the root cause of someone exhibiting the Dunning-Kruger Effect. We've all at some point worked or lived with someone with real illusions/delusions about their abilities, and the root of it is ignorance. There's little we can do in our workplaces to mitigate these people. Word of advice from my experience: Never co-found a vc-backed software startup with someone that's done genuine innovation....and been completely ignorant and oblivious about everything else.