The reason for this is pretty simple: necessity.
~~Companies~~ Corporations have a fiduciary duty to maximize profits for shareholders.
If no union exists, that means depressing wages as much as possible while meeting staffing needs.
If a union is forming, it means spending as much as you need to stop it since, if you don't, you'll be unable to depress wages over the long term.
When a union exists, well then they have to negotiate to continue operations and so workers get paid more fairly.
Join or organize a union if you can.
