Unions are primarily useful for workers who don't have the ability to negotiate their salary before accepting a job. By banding together, they're able to negotiate wages that more closely align with the value of their work. Knowledge workers are expected to do this for themselves on an individual basis as they're not really interchangeable.

I do wish people pushed for more democratic decision-making in their places of work, though. I've read that in Google's early days people were mainly promoted by peer-evaluations, and there was a mechanism to remove a manager from power if they lost the confidence of most of their subordinates.