[flyby]

Ignoring administrative details, a union is simply enough usually self-interested people becoming sufficiently one in purpose for long enough for it to be in an industry's best interests to cooperate with that union of people - instead of self-interested individuals - in order to enlist the labor that industry needs to achieve its goals.

The reason it hasn't happened is there isn't sufficient denial of self-interest for unionized interest to achieve critical mass to become a singular force to be reckoned with.