I need to know what you are paid so I can balance what I am paid against what I think you are worth.
Closed salaries only benefit the other side of the negotiating table - the employers - who then divide and conquer.
Which is why they play upon the doubt and insecurity in people's minds that they are actually worth what they have negotiated.
It's the basis of why unionisation works and why it ensured that workers got their fair share of productivity increases. Right up to the point when unionisation was defeated and 'individualists' got to play their game. Since then the productivity gains have gone to the top of town.
Extreme individualism is always defeated by those who know how to share and work as a team.