"Is it possible US prosecutors give so few shits about justice?"

If you are part of a system and have power in it, you believe the system is good. You believe that everyone else in that system is also good. That's why you have prosecutors and judges just believing the police automatically. Nobody wants to be in a job that ruins innocent people's lives, so everyone must be guilty or the cognitive dissonance will run you out (if the others in power don't first).