Most people by default tend to think they can do things better because we're shallow creatures and only look at the surface of those things.
What does it mean to be in a position of power? To gather all the benefits, decide who does what and what goes where. But do we see the competition, the rivalries, the sabotage in pursuit of one-uping the other? Do we see see the pressure of being in the center of attention at all times? To carry the burden of responsibility and required to be a model for others to follow? And to know that at the end of the day, one can do everything right and still lose?
Not everyone can be a leader, not everyone wants to share in the burden of responsibility. Because when things fail, and they do, it's the leader who has to fall on the axe.
Yet these positions of power, of leadership need to be filled. Those of us who know better, stay away, because we understand there are few benefits in that kind of position if you're not going to profiteer from it. The scoundrels, the liars and the crooks instead are all for it, as they care only for the benefits and the power, not the responsibility. So they lie, threaten and cajole, or even kill for it.
So the question you need to ask isn't why they the leaders. It's why aren't you? Think deeply and you'll find the answer.
You're not a leader, I'm not a leader and neither are most of everyone else. We let others carry the burden and put up with their inadequacies because we ourselves can't and won't take their place.