I don't understand - there is a job market, and like most markets here is information asymmetry. Companies have more information about compensation than workers. In this situation, having less salary information enable employers to take advantage of employees.

We're talking about this https://en.wikipedia.org/wiki/The_Market_for_Lemons right?

Are you so sure about that? Think about the CEOs themselves as the lemons... a lack of transparency can lead to the same adverse selection pressure that trends towards "lemon CEOs" flourishing, and pay transparency can help align the incentives somewhat.