Why should people express trust in you by deliberately sacrificing information relevant to their own interests before you've proven yourself trustworthy?

As the adage goes... trust is earned, not given. I would not divulge my current/previous salary either.

Well actually, one usually builds trust by giving it in small pieces, showing that you're willing and able to develop a relationship with the other person. But the key word there is small: you don't place yourself at a large disadvantage until the other person has shown themselves to be trustworthy in previous instances where less was at stake.