But also it's because "compensation" is far more than "salary" - which seems to be what this question is really about.
They say that the first to say a number is at the disadvantage. Yet nontransparency can be even worse in compensation negotiation.
>"salary is only a small portion of compensation. Let’s say you and I both make $5000 a month ($60000 a year – make the math easy). But you have 2 weeks of vacation, and I have 4. But I took the lower-deductible insurance option, and you took the higher. Which one of us is bringing home more per month? Who cares! My individual desires and needs are, apparently, being met on my package, and yours are with yours." (https://antipaucity.com/2013/05/21/publicizing-compensation-...)
I've written about this a few times on my blog and on Quora.
In short, "your" $90k and "my" $80k are different for more than just the money. Maybe I have more vacation, or better 401(k) contributions, or any of a number of other factors.
It would be great if employers would list their vectors of compensation in their first offer to you (https://antipaucity.com/2013/03/25/what-to-offer-to-be-the-b...) - but what I get offered, and what you get offered for the same position will probably be different: our experience is different, so our value to the company will be different.