I mean, look a the relative population sizes of the countries as well. Couple that with a much more armed populace. The police carry guns because something like 35% of the US population owns or has access to firearms. Meanwhile the majority of private gun ownership in the UK is in shotguns, with just a bit above 500,000 non shotgun certifications having been issued. That's less than 1% of the population of the UK having access to a (legal) non-shotgun firearm and at most 2.5% having access to a shotgun.

Then there's the argument of illegal gun ownership, cartels, and gang violence. To my knowledge, the US is faced with more severe drug related crimes, be they cartel kidnappings, murder, turf disputes, home invasions, etc. Incidents like the North Hollywood Shootout, in which the entire police department was so outgunned by two civilians they had to send officers to a gun store to obtain weapons capable of piercing the armor worn by the robbers, have left a serious impression on the police forces of the US.

I'm not here to argue whether or not they should, but have instead tried to offer a reason why.