First off, yes, not every situation is going to be the same, and sometimes you are 100% better off quitting.
But I have lost track of the amount of posts I've seen of multiple people, whole departments, or more than half the staff quitting due to poor working conditions, pay, etc. THIS IS EXACTLY WHAT UNIONIZING AND NLRA PROTECTIONS ARE FOR. When more than 50% of the non manager employees are fed up and ready to quit, you already have the votes and or it wouldn't take much to convince the few that need convincing if you don't quite have the 50% or more to vote yes. No employer is too small or protected against their employees unionizing.
Think about it. In today's work environment and climate, the chances are you're going to get a new job that just has different kinds of BS than the last. It doesn't make sense to keep jumping from job to job if you're in a position where unionizing can easily be a reality should you call a union and get the ball rolling. You're not saving time by quitting, you're wasting more time by all the times you have to job hunt for new jobs every time one is shitty. You're not saving yourself a headache, because you'll just have a new kind of headache at the next place.
YOU ARE LETTING THEM OFF THE HOOK WHEN YOU QUIT INSTEAD OF UNIONIZING WHEN YOU'RE IN A SITUATION IT WOULD BE RELATIVELY EASY TO UNIONIZE.
What do you think hurts them more? Quitting so they have to find new employees to treat like shit? Or staying, unionizing, and now they have to treat everyone better, pay them better, give them better benefits, etc, for the entirely of that businesses life afterwards. You're not only securing yourself and current coworkers better everything, you're securing it for every single employee that comes afterwards as well.
TL;DR
Stop quitting when you're in a situation where you already have, or it wouldn't take much to get 50% or more employees to vote yes to better working conditions, pay, benefits, etc. You are doing the employer a favor by quitting. Unionizing is the only, and biggest punishment you could give to an employer who has pissed off enough employees that it's relatively easy to unionize. Stop allowing them to treat the next batch of people like shit by not taking relatively easy opportunities to unionize. PLEASE!