I am currently working for a family owned business that is about 1.2 km away from my home. It doesn't pay as much as a larger publicly traded company would pay for my qualification and experience. If my potential new employer would know about my comparably low income, he will have a much harder time paying me significantly more or even just the usual market rate and negotiations would be that much harder. Due to other effects it could even lead to him questioning my competence, an often observed behavior. (there are other ways he could find out, but that is another discussion).

Frankly, I am not that interested in what others are payed. This is something I can negotiate with my boss myself and isn't my business for other people. And I don't want others to know about my income either. You can be sure that people will judge you by your income and feel superior if they make even 10 bucks more. Even your friends might get jealous about your income.

Some people see a value in discretion and some people do not. I belong to the former. There is just so much baggage attached to that shit that it isn't attractive to me at all.