> Last I recall, creating unions is an attempt at monopolizing labor to extort the employer. When this tactic is applied to any other factor of production it is called a “cartel” and engaged in “market manipulation.”

This seems like an argument in bad faith but I'll bite.

You are ignoring the pre-existing power imbalance between employer and employee, where the employer holds authoritarian control of production and the lives of employees. Cartels and market manipulation are mechanisms used by those with existing autocratic power to grow and strengthen their control over production and people, while unions try to balance this power by giving workers a seat at the table. I think it's pretty obvious that they are not the same thing