That may be because often trotted out as a convenient solution to any sort of wrongdoing by a lot of privileged people who themselves have their pick of jobs in US companies.
It's a convenient null hypothesis that could be the substitute for any remedy in this guide. Harassed by your boss? Quit. Company you work for is engaging in wildly illegal stuff? Quit. Facing racism? You guessed it.
That's not to say that quitting is not a convenient and probably the easiest option for a lot of people. I have used it myself. It's just not the one this guide is meant for.
So I think that asking for a "when to quit" part of this guide would be like asking for a "when to know you should just give up and buy a new car" section in a car repair manual [1]. But if you want a lot of people to be good at repairing cars, I would hazard that including a "when to quit and just buy a new car" section wouldn't be high on your list of things to add.
As for the concerted action bit, I have to agree - adding them would be good. My guess is that it would be unionization lite, and those are a non-starter in the US for a million reasons.
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[1] I have to admit I am not particularly clear on this point in reality, because I have never seen a car repair manual. Ouch. Anyone here who has actually seen one? Make me eat my words!