What is the reason for the American discomfort with salary disclosure?

I always wondered this because it’s not the same in Russia and maybe it’s not really a trend here, but people are so touchy about the subject of how much they make, what they want to make, and what the person next to them makes…what’s the reason for this? capitalistic tendencies? privacy notions/individualistic notions? everyone for themselves mentality? I find that people often want to know what I’m asking in my job search and are so uncomfortable asking but I have no issues disclosing that information…in Russia, as I mentioned, at least where I lived, everyone knew how much everyone else was making, it was sort of public knowledge in terms of what professions garner what pay…do you have any insight into this