I think it's because mobility is easy in the downward direction -- if the banker and the lawyer chose to become the janitor and the homeless guy, they could do that easily.

However, the banker and the lawyer realize that they have more life choices available if they are nearer to top of the hierarchy, and this directly leads to more obligations -- to maintain their positions, they must fulfill more responsibilities.

More choice leads to more reproductive success.

Q.E.D.