This has nothing to do with transparency. Just because one seller sells something at a certain price does not mean another has to, and same for the buyer.
The pay not being able to be negotiated is a combination of it being a political decision and lack of demand relative to supply.
When the government really wants someone, they start negotiating too.
Edit: one notable example of how it is politically impossible to pay appropriate to get the job done right the first time is the rollout of healthcare.gov and various health insurance websites by state governments. They could have paid enough to attract competent programmers, but the government decided to cheap out. Then it was a political fiasco when the project failed, and they had to open the government wallet to pay for competent programmers. But if they had advertised market pricing for programmers in the first place, and transparently, the the job would have been done right the first time.