Being more specific - there may be transition periods where you may become more valuable (through training or whatever) at which point you have the opportunity to seek higher compensation. If one does not seek higher compensation, then one is trading their higher potential salary for other wants/needs (maybe they trade the enjoyment of their current work at a lower pay over the higher salary at another job).

In equilibrium and in an efficient market, the value of the labor provided is equal to the cost of the labor provided.

Of course, in the real world, there are inefficiencies and irrational behaviors, but to a first order - it's approximately correct.