Who said anything about monopolies? I though a union was just workers banding together and using their strength in number to negotiate better working conditions

Who said anything about monopolies? I though a union was just workers banding together and using their strength in number to negotiate better working conditions, better pay and so on? It's easy for someone with a sought after talent to say normal workers can just "quit and sell their services to someone willing to pay more", but that isn't an actual option for many many people.