The system can only process and accommodate so many people. The choice is to focus on those who are causing the problem, or severely punish the general population that crosses an arbitrary BAC threshold. A high risk, accident-prone, repeat offender may view a DWI conviction as one of life's little inconveniences and a chance to live off the county for 30 days. Any law or regulation aimed at human activity must have an element of reasonableness. It must recognize that there are always competing motivations that dictate human behavior.
Most people drive to reach a destination.
The purpose of that destination may be work, family responsibilities, maintenance tasks, socializing, or recreation. If this is at the destination end of their trip, they will inevitably be returning with some amount of alcohol in their systems. In modest amounts, this rarely causes a problem or safety risk to others. Most people recognize this and act accordingly, in a responsible manner. A zero tolerance approach to drinking and driving will not work.