So the fact that "company is frequently interviewing" only shows how much demand there is and how hard it is to fill a position with the right person. Once you have the right people on board, they can ask a lot more than what they get, because replacing them can be significantly more time and resource/money consuming and then it's not even guaranteed that they will be as good as the ones you have right now.
This is basic free market, the side of it which has the upper hand right now needs to play its cards to max out wealth and every employee should be encouraged to do so, because it will overall only increase economic growth. It doesn't always has to come from business growth, if workers can gain significant wealth based on the same principle then they should, because it will only increase more spending and investment of those people into new areas again.
I loved your comment about "max out wealth" because that is the underlying truth to all of this--you get maybe 30 years to make as much money as possible and that's it!
This is of course for people that value what they do at work and want to make meaningful impact professionally in their lives. For other people family is much more important so it doesn't matter if they don't min/max the projects they work on.
That employees "pay that fee" doesn't indicate why. It also doesn't provide insight into whether it makes economic sense for them to do so.
This works both ways.
The value of internal knowledge helps both parties, because as an employee you dont know if x other company is good to work at, or that you would like the responsibilities there, etc. The fact that lots of people stay with a lower salary in a job is a testament that it is often the employees that choose to pay that fee.
The company is frequently interviewing. So if the company finds a candidate that would work for less (minus the internal knowledge comission) it should fire you and replace you!