In a normal publishing environment, if there was a publisher that was highly profitable and moving up the sales charts, one of the other companies would move to acquire the rival, thereby capturing both the new profits and the new catalog.
With Amazon, that doesn't work. With KDP, none of the Big Five can move to acquire Amazon and Amazon isn't going to sell even if they could sell. In order to absorb the threat of the self-published author, the Big Five must court all the successful self-published authors. They can't do this in one fell swoop. They must do this one author at a time. That's expensive, but ultimately worth in. Inexplicably, the Big Five use their author acquisition system to get authors, which represents a loss to a successful self-publisher. In order to acquire a self-publisher, they must acquire those rights and brands as if they were acquiring a company, which doesn't work because the author loses too much money, or even worse, the author starts a new pen name and continues as a self-publisher, which only perpetuates the circle of competition.