Unions don't require graded pay scales. You don't have to be a miner to be systemically taken advantage of. If one employer offers an unfair employment contract, you can try another company. If every employer offers an unfair employment contract, you don't have the option of another company. I can name a few clauses that favor only the company that are present at every software employment contract in almost every state.

There are more potential employees than potential employers, and most companies will survive understaffed longer than US laborers will survive unemployed. Inherently, the negotiations favor the employer. The more output the employer has stolen from its workers, the more of an advantage it will have. Unionizing and collective bargaining balances those scales.