People are lazy. Calculating seniority is easy and "objective", not controversial; in fact, it's so linear you could even write a program to automate it (I bet somebody did just that). Evaluating performance or actual returns to the business, that's hard; the metrics are often very subjective, and people will start arguing. Rule #6582 of the Safe Manager: Don't Give People Excuses To Argue. So seniority it is.

This was unfortunately exacerbated by Trade Unions, which needed something "objective" to force owners to pay up and keep members happy, so they built their demands on this sort of easy metrics: seniority, hours of work, etc.

I'm sure if you could find a generic way to calculate productivity in an uncontroversial way, managers and unions would jump on it, but in most cases there ain't, so seniority it is.