Whereas "Accept our terms of employment or starve," isn't?
Which is indeed the case in the short term. However, quitting your job means not making your rent payment for a quarter of America, so the analogy still holds.
Management bargains collectively - why can't their employees do likewise?
Inside the company, management's position is highly collective. Owners expend significant resources to get the best possible deal on what they have to pay their employees - resources that are beyond the reach of an individual employee. For jobs like this, it imposes pay grades across the entire warehouse floor - as a take-it-or-leave-it proposition.
It only makes sense that employees should organize their own advocacy groups.