I think it has to do with oversight. There's a strong precedent for the international community to hold a country's leaders as responsible for their military's actions. The chain of command reflects that.

Compare that to internal affairs in law enforcement. It's a terrible joke.

There are also fiscal pressures, in the way budgets are set, and with civil asset forfeiture. The US military budget is not set by how many kills they get, and units are not permitted to pillage and plunder the countries they invade.