They don't control squat. The unions and discipline review processes do, and they are unchangeable, so they need to be replaced with something that serves the needs of taxpayers and the community.
Say you are a reform minded mayor. You are stuck with a review process codified with your agreement with the cops. You install a Chief that wants to end choke holds. The problem is that the discipline review process relies on precedent. The Chief "outlaws" choke holds. Some DB officer keeps choking people. They try to fire him. He appeals to the mandatory review board that rules that he can't be fired because for the prior 40 years other cops were choking people and didn't get fired. It's the same story over and over. There IS NO reform under the current, precedent reliant system. That's the point.
Defunding the police doesn't mean ending law enforcement. Sure, it sounds scary and is an especially effective political message with people that that like to be scared. In reality, It's an end around against a system that BE DEFINTION prevents the reform the status quo lovers pretend to favor. It's a way, the only way, to break the bureaucratic barriers to improvement.