Because that information has benefits for labor as a whole. Conversely, "I want my salary to be a secret" isn't a good reason to keep it private and I don't see any good reason why it nerds to be private. Nobody is asking for what you jack off to, because it isn't relevant for salary negotiations and it's actually something that's reasonably expected to remain private. Nobody is asking about the rest of your life. Why are you so defensive?
I am not “defensive”. I am merely pointing out that you don’t have any right to know any particular details about me, including my salary, no matter how “useful” you consider it to be.
And you don't have the right to absolute privacy and secrecy just because it's more convenient to you, especially when they're extraneous things like your salary and not your private life. Ridiculous.