I make a lot of money because I studied hard and learned a valuable task. A leads B and B leads to C. It’s all nice and neat and comforting. Work hard and I can get what I want.
But really, is the world that orderly? Does the hardest working or strongest or smartest always get the prize? There are many people who work hard at a job that is less valued in our society therefore earn a lesser salary. There are many strong people who never quite seem to find the right situation and flutter from job to job looking for that elusive break. And there are many smart people who are unable to find work that satisfies them, and they end up pontificating without an audience.
Survival of the fittest is not fool proof. At all.