Because there's hundreds of nuances and circumstances that can affect how much a business needs to pay someone to secure their service, and not all of it can be compared apple to apple.
Real world example: I'm paid almost twice the market rate for my current job. Why? Not because I'm better at it than my peers, but because when I started, I opted for more RSUs (public company) instead of base salary, and the company has grown tremendously. Folks who didn't take the same gamble have a much lower total comp now. Years ago no one knew this was going to happen, so my base salary was lower. If the market hadn't shifted the way it did, should I have been able to complain others are paid more than me? Seems like a gamble with no drawback which is a bit unfair for the others, no?
Some folks opt for more time off. People with the same title end up at a different place within the salary bands based on countless factors, many of which are very subjective (and if you put too many titles, then people will just negotiate the title instead of the salary, effectively doing the same thing).
People are also generally biaised when talking about themselves and aren't very good at fully understanding the context around the salary of others.
All these factors mean that an objective salary based purely on quantifiable factors won't end up 1:1 with how good people are at their job. Thus we have negotiations.
And then there's environmental factors. If companies aren't allowed to negotiate, you end up with a situation like this:
Company Foo pays 150k for a Sr Eng. Bob is paid 160k at his current job, and Foo is trying to poach him. Bob asks Foo for 165k. Without negotiations, Company Foo can either:
A) Tell Bob tough luck (which in the current market is a bad move. Software Engineers don't grow on trees)
B) Give all their Sr Eng (potentially hundreds or more) a 15k raise. But then the same thing will happen to Company Bar... When does it stop? Should everyone at all tech companies but paid precisely 150k? What if Company Foo and Bar are very different and why? It's very subjective.