Some workers get paid a lot more than others. A few CEOs make tens of millions of dollars per year. Other workers get paid $5.15/hour, and some argue that this legal minimum is too high.
There are several significant numbers here:
V is what a worker gets paid
W is what a worker contributes to his employer in the job he's currently in.
X is what a worker contributes to the economy in the job he's currently in.
Y is what a worker could contribute to an employer if his job was perfectly matched to his talents.
Z is what a worker could contribute to the economy if his job was perfectly matched to his talents.
In a perfectly free market for labor, every worker would get paid slightly less than Y. (Slightly less because we expect the employer to make a profit.)
In a world better than a free market where there were certain government regulations in place to prevent externalities and rent seeking behavior, every worker would get paid slightly less than Z. This model maximizes the nation's output.
In the real world, V is only very modestly correlated with W. Two workers might be at the very same company and contributing the exact same amount to their employer, yet they get paid vastly different salaries.
What's the difference between W and X (or between Y and Z)? One example is a salesman. He contributes to his employer by selling his employer's products, but he's not contributing to the economy at all because his job is a zero sum rent seeking job. Or even worse, if his employer's products are worse than competing products, every sale he rings up actually has a negative impact on the economy.
Why are worker salaries so vastly out of sync with Y? Here are a few reasons:
(1) Lack of information. Workers don't know what they are truly worth because they lack information about the labor market. Most people don't even know what their coworkers are getting paid! At the same time, employers don't even know how much their employees contribute relative to other employees and they have even less of an idea how much potential employees might contribute if hired.
(2) Unequal bargaining power. I've written before about unequal bargaining power. This almost always benefits the employer but in rare cases it benefits the worker.
(3) Management problems of large organizations. A manager makes decisions to benefit himself and not his employer's stockholders. A manager may hire a pretty woman rather than a man or an ugly woman who would do the job better. He may purposely hire someone not so good in order to make himself look better. My post about whether companies really want to hire the best employees is relevant here.