Humans are tragically limited in some literal and obvious ways. For example, most information comes in through 2 very local light senors and a pair of ears, and is then processed by a gooey grey lump that is very, very biased towards the present.

Compare that to the problems that require leadership, which often span large areas of both time and space. Obviously, no human is up to the challenge of comprehending the actual challenge.

A leader who actually believes they understand what is happening is delusional. Not necessarily in a bad way, they just aren't engaging with the reality of the situation. They will hopefully still get buoyed up by their other skills and the work of the people around them, and with a little luck and skill might have a grasp on a couple of the key drivers of a given situation. The true state of things isn't going to stop leaders pretending they have an answer to everything, because leaders are chosen by the people and people prefer to be told that the situation is easy and under control than the messy reality that requires caution, risk management and contingency planning.

I've yet to see the public admit en-mass that they are bad at picking leaders, but they clearly have no idea how to. If we had a better system for choosing political leaders we'd certainly see it used.