I think some of this is cultural; it's often considered in poor taste for one to share information about personal finances with others.
It's also an issue of provoking others to jealousy and having to deal with the issues surrounding others' jealousy.
Ten years ago, I relocated to be closer to my family. As a result, I took an enormous pay cut. Prior to that, I taught in a well-funded school district that paid among the highest teacher salaries in my state. I had relatives that figured out that my income was high, and they often voiced their disapproval that I should be paid so much. (Since I did not work in the hot sun doing manual labor, they did not believe I should command a higher annual income than they received.) I suspect the reluctance of most people to share their income information with others is to avoid dealing with any judgement. (The reverse could also be true.)