Maybe part of the problem is every state and every police force has different training, structure, etc.? Why wouldn't this be standardised at country level?
Here in WA the state patrol has an academy where officers from various departments around the state receive additional training. This helps to standardize training around the state. It only helps.
At a national level they could perhaps train the trainers or high level officers. Not much more.
There are also overlapping jurisdictions. State level, state patrol. County level, sheriff. Local level, town or city police. Some sheriffs are elected.
Most recognized native tribes have their own police departments which have recently been granted federal recognition by the FBI and have never answered to local or state authorities although there is cooperation.
At the national level we have a zillion federal agencies, departments, and offices many of which have their own armed uniformed security services with their own particular agendas.
This article goes into the assorted federal police forces a bit.
Its about as clear as mud to me and I grew up here.