TL;DR When workers start out, the firms know a lot more about their abilities than the clients do (they have an "information advantage") as the worker has very few ways through which to prove their abilites. Over time, though, as the employee's public performance increases (through successful cases, good investments, etc.) the information advantage the firm has becomes lower. Eventually, the firm lets the employee go in order because the worker now has proof of their competencies that they can show to clients to demand higher wages directly.