Remember the Dunning–Kruger effect: Experts are full of self doubt, while fools think they know everything.
You see, to know how good you are at something, you have to know that thing well. And if you do, then you know where your knowledge fails you. You know of a great many more things that you could learn.
But the ignorant know nothing of all the things they don’t know. So they foolishly believe that they know everything that’s worth knowing.