The wildcat strike by teachers in West Virginia is a good example where simply quitting wasn't effective.
Your comment "get you more money" makes it seem like you think more money is the primary reason people join a union.
People also join a union to have better defense against capricious management decisions ("Give the superintendent's kid an A or I'll give you bad teaching assignments".)
People are also willing to take a lower salary for better job security.
Grievances can be mediated through a union rep, like the shop steward, who is more likely to be aligned with the employee than the company's HR rep or management.
In some companies, unions have right to have a member be present at board meetings.
> where quitting is normalized generates similar benefits to unions
That's somewhat tautological. The power of unions is based on people quitting. The point is the quitting together is more effective than quitting piecemeal, such that even the risk of having mass coordinated quitting can be an effective tool for the union.