For the same reason in the private workforce an employee is worth more with more experience. For example, an entry level engineer will start at a entry level engineer pay. But 5-10 years later, if he leaves the company to go to another company, he doesn't get entry-level pay at that company. He gets 5-10 years experience worth salary. Unfortunately, the unions have made the seniority system as hard lined as it is today, and I don't ever see a national longevity list.