Is there an economic reason why employees don't share salary information with each other?
dwjohnston,

To answer your question briefly: no, because it is rather a social or psychological matter than it is an economic matter.

There is already asked a good question about this subject in The Workplace community: https://workplace.stackexchange.com/questions/15397/why-is-discussing-pay-such-a-taboo-how-can-i-get-people-to-talk-about-concrete/15431#15431

Feel free to read the (also good) answers. The answer section also contains some handy tips and tricks that you and your co-workers could try.

I hope this will be useful for you.