Becoming a millionaire takes time and requires some risk-taking. If you start out non-rich, don't expect to be rich before you're 50. McKenzie and Lee point out that the process of becoming rich normally takes decades, sacrifice, dedication and perseverance. Becoming rich is not for everybody. Some people quite rationally may not choose to make the sacrifices of current consumption and enjoyment.
Nothing's wrong with that.
What the authors stress is that it's one thing to make a conscious choice not to pursue great wealth, and another not to know the choice exists. McKenzie and Lee demonstrate that greater wealth is possible for most Americans. That's a vital point, particularly for black Americans whose political leaders preach that they can't make it because of discrimination. By knowing where a person ended up, it's a risky proposition to speculate about where he began.