Unions don't like competition. Having a monopoly on the workforce is how they have bargaining power because they don't bring any new value to the table for the buyers of labor.
Sure they bring value, they act as a force multiplier and save individual workers time spent in tedious negotiations where there is a huge asymmetry between worker and employer. It's just delegating an administrative task to a nominee, like being represented by an attorney in any other sort of negotiation.
