There are historical reasons, like the fact that LE was created to protect a special class), which deeply impact the way LE is trained, which dictates how LE interacts with normal people. There’s what you suggested, that they want great stats, because political careers are made off of stats, and winning high profile cases (there’s a long list of questionable cases where DAs railroaded a person or group just before they ran for governor or congress.
The book “Unfair: The New Science of Criminal Injustice” discusses a lot of the failings of the system, and gives suggestions on how to fix the system without tearing it down. In discussing the failings, he gives a lot of thoughts on why things are as they are, etc. It’s a pretty easy read.
In case you missed the link on the origins of police somewhere else in the comments: https://worxintheory.wordpress.com/2014/12/07/origins-of-the...