These are realities. However, the U.S. offers opportunities for all people that exist nowhere else in the world. Many people go back to school later in life to learn new skills to prepare them for better paying jobs. This is healthy and good for the country and for people. The United States has never been a static economy or had a stagnant workforce.
People constantly change jobs, and occupations.
The opportunity to earn learn more and earn more is there for those who take advantage of it. The fallacy comes in when an attempt is made to compare different occupations. It is impossible to do this. Some women's groups have tried to prove a pay gap. The best way to 'equalize' pay for men and women is to encourage women to go into more nontraditional jobs.