Seniority lists are a very real oddity in the modern world - but then so is the job TBH
very few jobs require exactly the same level of performance every day for the whole of your career. And even fewer that you perform the same tasks with a different set of workmates every day
The possibilities of promotion (in the normal business sense) are very very few and most people don't even want to manage others (outside the cockpit)
The companies don't want a work force that has a range of abilities - they want one where everyone has IDENTICAL abilities as far as possible
If people are happy with seniority I guess it's as good a way as the next to manage a weird situation