Yeah, you handwave some of the most visible and effective unions in the United States (SAG-AFTRA, WGA), you're also ignoring education unions, healthcare unions, and many other careers where pay is not just about seniority, but also education, certification, specialization, and other factors.
Even unions in industries with legitimately undifferentiated labor will have shift differentials built into their contracts.
Even unions in industries with legitimately undifferentiated labor will have shift differentials built into their contracts.