Money, politics, and fear that's the reason. Money because a bunch of different people make a lot of money screwing others this way from drug testing centers to lawyers to courts to dmv to treatment centers to insurance companies and probably more. Politics because it wins political races to be tough on crime and to have a history of supporting legislation that's also tough on crime. "Tough guy" after "tough guy" things just get worse and worse that's really the only way it can go until people have finally had enough. Fear, people can be made to fear anything and fear is also one of the strongest political tools.
Or the short answer is because we allow it to happen. Soon we'll see chain gangs come back and you'll end up as a slave laborer for years just for simple posession. Didn't Arizona just try chain gangs and stop that again just recently?