We have to distinguish here between a single, centralised police force that handles all levels of policing across the country (which is what most commenters rep

We have to distinguish here between a single, centralised police force that handles all levels of policing across the country (which is what most commenters replying to your comment seem to be debunking), and having a single, centralised "federal police" force that handles most law enforcement for a single layer of government (which is what I suspect you're getting at). The US is particularly unique in that even amongst countries with relatively decentralised law enforcement, I can't think of another country where most major government departments have their own internal police force ("why is the Department of Education buying shotguns") or so many "primary" federal law enforcement agencies (FBI/DEA/ATF, etc) which would normally be much more consolidated.