Two reasons: 0. Military Indu$trial Complex 1. Moral-panicking Christian sociopaths hate poor people
Pointing a finger at a single group is unproductive when the desired outcome is a more comprehensively understanding, less punitive, and essentially more humanitarian solution for what is at issue.
The real disconnect is that the people in power are not held to the same standard as those being sentenced. When was the last time you heard of a prosecuting attorney being drug tested before they could argue in court? What about a judge, or probation/parole/police officer? If there was some transparency in how ubiquitous drug use actually is, one might have some real leverage when seeking to implement more equitable (and therefore less punitive) legislation that provides fairer sentencing guidelines.