Why should a person be promoted over someone else purely due to a start date?
Because several (hundred) aspirants for the one position might have similar experience.
Why should it not go to the better suited applicant?
If several applicants are similarly qualified, it probably will - or to the bigger brown noser, or the one willing to work for less than the others.
Does flying skill, experinece (on type and other), professionalism and airmanship not come into?
Yes, as well as previous employment history, number of employers over a defined period, incident/accident record, recommendations, peer worker compatability, etc.
No it seems - just based on a date, full stop.
That has
NEVER been the sole reason for promotion, even with seniority based systems.
Even with post 1989 Ansett, length of service in the company was used as the basis for future promotion - which suited the Heros just fine, once they`d secured their place by other means.
S G