That is the worst thing you could do to law enforcement for two reasons. The first is that it would make law enforcement entirely unaccountable to the people being policed. It would effectively turn all police into the FBI. Being real for a moment, how easy do you think it would be for you to get the FBI to change a policy you disagreed with? What would you do? Make a complaint to the head of the FBI in Washington DC? Write to the President? San Francisco is a perfect example where an errant course of action was changed due to mayor London Breed being local and directly accountable.
The second reason is that the whole law enforcement apparatus becomes more brittle do to the inflexibility of having a single point of control. Distributed systems are more resilient, and law enforcement is no exception. If the law enforcement is uniform, then there is no escaping bad policing because it is the same everywhere, and I guarantee you it will never be the perfect utopia police you have pictured in your mind if only you give total control of it to the Executive Branch in Washington DC.
We have so many layers of law enforcement in order to provide more accountability to those being policed. We have a Federal Police, but States are allowed to set up their own and they'll have jurisdictional control if they do. Counties are allowed to set up their own and they'll have jurisdictional control if they do. Towns are allowed to set up their own and they'll have jurisdictional control if they do. If a town doesn't choose to set up their own police then they can use the county law enforcement agency. If a county doesn't choose to set up a law enforcement agency then they can use the state's law enforcement, and if a state decided they didn't want to do law enforcement then they could use the federal's law enforcement. The whole system is designed to be as locally controlled and locally accountable as those being policed want it to be.