Why Don't Wages Fall So That Everyone with the Skills

Why don't wages fall so that everyone with the skills and desire gets a job?

A) The government might prevent it, through minimum-wage legislation.
B) Labor unions might prevent it, through bargaining backed by the threat to strike.
C) Firms themselves might prevent it, by voluntarily choosing to pay higher wages than necessary.
D) All of these are reasons why wages may not fall to equilibrium.