Most people simply don't negotiate any part of their job beyond initial salary. Sometimes people might negotiate benefits, but that is somewhat rare.
More importantly, unlike traditional contracts with expiration dates, the average person doesn't renegotiate their deal very often or at all. They take what they get from their boss, hoping to get at least a cost of living increase, which is more and more rare.
All of this means that each year you are effectively earning less if you don't renegotiate your deal. What was maybe a good deal 5 years ago is now worth 11% less than it was when you started.
So, unless you are renegotiating your deal, you are agreeing to keep a deal that is less valuable each year.
Knowing that, having an agent or any mechanism to force negotiating on an ongoing basis would be a huge step in the right direction for most people.