[mcherm]

There are two reasons.

First: because it is socially unacceptable.

In the US culture, talking about one's salary is taboo. It is rather as if someone were to post naked pictures. Sure, you are an enlightened person and you probably wouldn't hold it against them. Much. But you'd find it a bit weird. Americans try hard to hold to the illusion of a classless society and treating everyone equally, and hiding salary is an important way to do this. It also allows people to not feel cheated, if they don't realize that others are making more than they are.

Secondly: because my company might not like it.

US labor law actually REQUIRES that employees be permitted to discuss their salary with each other, which is why companies do not forbid discussing it, and to be clear: my company does NOT have any policy against discussing salary. But it is to the company's advantage to be a bit vague about salaries. Cynically one could say that this permits them to pay each individual the minimum they can get away with without having to deal with a sense that this is unfair. As a representative of my company, they would probably prefer that I talk about work conditions and company culture rather than specific numbers about the salary we pay.