I see what you are asking: Why doesn’t market competition drive down prices?
People have to publish in prestigious journals to make a career in science. So, that’s the “service”. Their position in the system lets them extract payment for something that other people deliver.
Even if someone opens up a competing journal, they are not likely to get quality submissions, because publishing in some unknown journal does not help the CV.
At the same time, the cost is mostly born by other people. Librarians pay for the subscriptions. I’m not sure why there is not more pushback from that angle. Eventually, institutions need access to these journals.
The cost to scientific research is spread over all society. No one person feels it. No one can even be sure how much better things would be under a reformed system.
Progress happens only when someone goes too far and causes outrage in the academic community. There has been some progress to move to a better model. But all that money can pay for a lot of PR.
ETA: On second thought, I’m probably simply not aware of the efforts of librarians, etc.