Not every employee is a good negotiator. And poor negotiators get underpaid in a market system even when they are great at their job. Unions fix this market inefficiency by having good negotiators act on their behalf.
In the good old days companies paid strike-breakers to crack skulls and sometimes kill employees who agitated for better working conditions. It is because of collective bargaining that workers got a 5 day work-week, mandatory holidays, sick leave, maternity leave, a basic pension, and the right to a safe workplace. Many have died to ensure these basic rights for everybody.
In the good old days employers didn't have to take factory safety into consideration. If you got crippled by factory machinery you would get fired and live the rest of your short life in destitution. Let's go back to that system. Employees will be free to choose the factory that maims them. The cost savings will surely be passed on to the consumer.
How else do you want to balance out the power distribution?