I think that relates in part to how big the disparities are. If people are making mostly similar amounts of income, risk of discontent may not be as big an issue. For a large-scale example, Norway makes the annual income of the entire population public information, and yet the country isn't erupting in discontent. Of course other elements of culture (whether national or company culture) are involved as well.

I think it's reasonable to believe that the employer should have a good answer to the question "why is s/he making more than me, what does s/he do better?"

Yes, but let's not be naive enough to assume that people would be satisfied even with a good answer.

Well, a good answer would be "because s/he is familiar with codebases A, B and C and has skills X, Y, Z", and I would expect the employer to follow-up with a raise if the employee in question also learns all of the above.

Or naive enough to assume that there is a good answer beyond "You're not a good negotiator so of course we're taking advantage."

There's always going to be discontent no matter what. If someone thinks they're worth more, you can either agree and pay them more or disagree and they can leave. That's how a market works. Your employees almost certainly already know more or less what they're all getting anyway, and most of them are probably unhappy about it. They keep showing up and doing the work because it beats the alternative for them.