Certain markets are highly competitive for labor. A good example is software engineering. In those cases, because neither the corporation has a monopoly on their tech products nor does a union have a monopoly on labor the sector can be highly productive and everyone can get rich. In such a situation it seems pointless to have a union. Why would an employee want to be beholden to both his employer and his union? Unions, as well as giving their members some negotiating power, can also limit and burden their members in many ways.