One of the terms unions love to have is a requirement that all employees be members of the union.
When you have to pay the union in order to keep your job, the union becomes a tax on your income... and the union has no incentive to treate you right-- as what are you going to do? Quit and go work for another company? That's your only option.
That modern unions attempt to force employees to be members when the employees don't want to-- shows that the unions themselves know they aren't offering an advantage to the employees that the employees would voluntarily choose.
Walmart doesn't force me to buy its hot dogs. Walmart competes with other hot dog providers for my business. (And loses because there is a very good german deli in this town.)