Would realizing they are lucky benefit the lucky? In the cookie experiment Lewis wrote about, the randomly appointed leader would then not take the cookie so effortlessly.
He's telling the students that they, being Ivy League graduates, will have many opportunities to exploit their positions, and is telling them to be aware of this and exercise some humility by remembering that luck has largely brought them where they are.
Indeed, and that might result in better relations with the other members of the group, in turn resulting in more productivity, leading to greater accomplishments and benefits later on. It's been thousands of years since short-term greed was the ideal way to do things, basically since humans organized into real civilizations, but it's hard to get people not to be greedy, so we just design our economy around it instead (i.e. capitalism).
