Unions periodically renegotiate their salary and benefits through broad company-wide contracts, so its not just a single small bean asking to be paid a bit more. Its an entire department or firm demanding a bigger percentage of the gross revenue.
Seniority solves a lot of the "how much money do I even ask for?" questions when renegotiating salaries. It also establishes a clear-cut cost of living track. You know what you'll be making in five years, so you can buy a house or get married or have kids with some underlying expectation of how much you'll earn when your cost of living goes up.