Employers already can pay for this data through companies that literally provide this. Employees do not have access to the same data due to cost (unless you count stuff like levels.fyi or Glassdoor). How would making this info more public help employers more than employees?

Glassdoor doesn’t count. It’s all self reported and full of biases. Taking a look at the median salaries for a few positions in SF and it’s grossly too low. Plus a lot of people might have the same job title as you but it doesn’t mean they do the exact same job as you