No, there're other arguments as well. One of which is privacy, for example. Many people value their privacy more than some elusive goal of full transparency.
If you consider that private information. Plenty of people, inside the company and outside, know that information. It's not that personal in my opinion.
You can even reasonably guess someones salary based on their job and location, especially a coworker's.
The same could be said of much private medical information. Guessability, or someone else knowing the information, are not the controlling factors in what is considered private information.
That doesn't mean it's not private. Lots of private information is known by lots of people. Your health record is known by your doctors and the government presumably. Government knows everything about my taxes, my date of birth, national ID (or SSN in US) etc. That doesn't mean it's not a private information. I don't want to share my salary details with other people unless I choose so.
There're plenty of ways to learn of one's SSN or birthday date too - this is still private information, which doesn't concern other people, unless the person explicitly wants to share it.
Most privacy concerns have to do with information that could be harmful in the wrong hands. My web history is useful to me, but if my roommate gets ahold of it, I will be embarrassed to no end. My SSN ensures I can identify myself within America's bureaucracy, but if someone else has it they can open credit cards in my name. My salary lets me make a budget but if someone else knows what my salary is...then what? How does that information harm me when a third party gets it?
- an insurance company demanding a premium because you're well paid
- competitor bidding for your labour anchoring their offer to your current salary
Those are just two cases were it could be harmful.
The idea is to make the information available and transparent while still not making it public and still protecting it.
I'm not a bad actor, who spends all their time figuring out how to make profit off of someone else's private info. I'm pretty sure there're very creative ways private information (salary and not) can be misused, but I don't want to find out by volunteering this information.