Occam's razor answer: Because the vast majority of Silicon Valley engineers are happy enough with their working conditions, with their ability to find new work if their current gig goes belly up and confident that if a major conflict occurs, they can find the help they need (eg. a lawyer) in the open market.
While that's probably true and I think people should be happy with whatever makes them happy, it doesn't mean these engineers couldn't be much better off.
That being said, as an employee, I'm so afraid of unions screwing up that I can't make my mind between having an union vs. these periodic lawsuits/settlements in order to keep the balance. It seems every example of a union being a good thing (like the Norway example mentioned here) feels like a bug in the matrix.
