There's a tremendous amount of idealization involved there -- that the cost of renegotiation/finding a new job is zero (it's not), that no systematic effects ou

There's a tremendous amount of idealization involved there -- that the cost of renegotiation/finding a new job is zero (it's not), that no systematic effects outside the market exist lowering salaries (they do), etc. As an approximation, employees are paid what they're worth. It's an approximation, not definitional.