system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have over 18,000 unaffiliated police departments in the United States? Why isn't there a single government agency that unites them?

I guess I am asking why we have no national police force.
gpt
Because there are lots of levels of legal authority in the US, and they are largely independent of each other (by Constitutional design). Federal laws are not the same as state laws, which are different from county or city ordinances. And sometimes they conflict. No single agency has the authority to enforce all those different laws, and even when the laws coincide, the heavy lifting is often left to the lowest level of authority that can effectively do it. For example, a murder investigation is generally done by the city PD where it happens, because they can do a local investigation best... the case will then be prosecuted by the locals _unless_ it's part of a bigger batch of crimes - like a multi-state crime spree - that needs interstate authority, and then the FBI or some other fed agency gets involved. 

For the most part, a state trooper or FBI agent isn't going to write you a parking ticket. And a county sheriff's deputy isn't usually going to be running a murder investigation. Now, if a cop witnesses you breaking the law, they may be able to hold until the proper authorities do arrive, but maybe not. Each of those areas may cooperate with neighboring areas, but they might just as well jealously guard their independence.

A good example is the recent dustup in the South & Southwest over using local cops to do immigration enforcement. The short answer, from a legal perspective, is: they can't. State, county, or local cops are not authorized to make immigration busts for the simple fact that states, counties, and cities don't make immigration law. Only the federal gov't can do that, so only the federal gov't can enforce those laws. ICE may sometimes use local cops for additional manpower on larger raids, but the busts are all ultimately under the authority of ICE (or whoever).