What you're saying is "companies should hold a lot of power over workers." Management is much more powerful than an individual worker and collective bargaining via unions seeks to be a check on that power. Why is that so undesirable?
Maybe the company thinks unions are inefficient or doesn't like the collective bargaining power (in extreme cases it resembles a monopoly). Companies and workers are entitled to their opinion. Workers can choose to work at companies that allow unions.
Of course management (multiple people) should be more powerful than an individual worker.