Because they have the ability to get things done. With potential leaders, when the work comes in, it's complete. The half-cooked meal isn't good enough. No one can lead without being criticized or without facing discouragement. A potential leader needs a mental toughness.
Leadership creates a certain separation from one's peers.
The separation comes from carrying responsibility that only you can carry. Years ago, I spoke to a group of presidents in Columbus, Ohio, about loneliness in leadership. But they never know which ones will work, so everybody who likes my idea jumps to work on it. Those who don't, work against it. Employees are going backward and forward-when the idea may not even come about at all.