I mentioned elsewhere that there are many industries where transparency works, and the public sector is one of them. Your employer wants you to deliver deterministic output, and your pay reflects that.

That's less the case when you're constantly challenged with clean-sheet exercises where every single person will produce a different outcome. It doesn't even have to be a super senior job - think of someone writing newsletters. That channel can easily lead to 5% or more of total revenue, and different people will definitely produce different outcomes (just the subject line alone will have an impact). Why pay different people the same rate if their work leads to vastly different revenue?