Obviously, there's one big reason: few of them could earn comparable wages anywhere else. But there's also the problem of worldview. Management's fractious relationship with the unions over decades had built up a huge reservoir of distrust among the workers, as one of my commenters points out:
Cooking for yourself is one of the surest ways to eat well. Bestselling author Mark Bittman teaches James Hamblin the recipe that everyone is Googling.