Very simple. It's called "prior restraint."
If I drink six beers, fire up the car and go tooling down the road, I have not harmed anyone.
Yet, you say. True, but that's the point. And if I load up my rifle and step out into the back yard, I haven't harmed anyone. Yet.
At what point do you start locking people up for what they might do?
Do you lock up people and take away their children because they were abused as children? After all, abused children grow up into people that are at a greater risk for abusing their own children. Why not just lock them up now, before they get the chance?
Or how about banning men from any place that single women drink? After all, those drunk women might decide to go home with one of those men, and then he'd be guilty of rape. Better to lock them up.
Or how about locking up any driver over 65 found driving at night. It's a medical fact that night vision deteriorates with age. Those people might be impaired. And we'll lower it to 60 in a few years. I mean, it's better to be safe, right?
See the problem? If you go around arresting people because they might hurt someone, you're going to have a lot of very full jails. That, or the law will be unevenly enforced because the police can't be everywhere. Kind of like it is now.
But it gets scarier. Right now we arrest people because they're driving drunk and might hurt someone. How long until some wacko congresscritter submits a bill to lock up anyone that is pro-life? Hey, they might go to a protest in front of an abortion clinic and frighten pregnant women away. We better lock them up because they might violate that woman's rights.
Locking up people because they might hurt someone or might cause property damage is prior restraint.
If the laws were sensible, then drunk driving, in and of itself, would not be illegal. But if you hit a parked car, drunk or sober, you'd be tried for vandalism. If you injured someone, drunk or sober, you'd be charged with battery, a felony. And if you killed someone, you'd be charged with manslaughter.
Whether a person is drunk or not should be entirely irrelevant as far as the law is concerned. Until every cop is trained to be psychic, there is no way to make sure that people that might hurt others get stopped. So how about we concentrate on actually putting people away that have actually caused harm? Instead of filling up the courts and jails with so many "might have" cases that the real criminals, the ones that have actually caused harm, get rolled right back out on the street to cause more harm. You know, like it is now.