Yeah but that in itself is useful information. It signifies that demand for those kind of employees is high and that should in turn affect the salaries of both new hires and employees bold enough to renegotiate.
The only reason it's not public, is because an inefficient labour market helps the employers underpay their employees.
If that were true, why wouldn't everyone voluntarily post their salaries publicly? For that matter, why won't you?
It's already protected for employees to share their salaries. If the only reason salaries are private is to help employers, wouldn't we expect many people to share their salaries voluntarily?
Culture rules. People are not far-sighted logical market agents.
Consider the opposite: If sharing your salary is so inoffensive, why do companies go to great lengths to stop you from doing it, necessitating enshrined legal protections (that most people are unaware about)?