I agree with you that it is not a crime by definition. But I think there should be a sufficient detriment to prevent people from doing this. And I think labeling it as a crime is the easiest thing to this end.

What is the alternative? Being frowned upon?

You say about how people automatically agree drunk driving should be a crime. But I always use to wonder how blindly we trust others on the road to behave responsibly. How often we leave our lives and our loved ones, to the fair judgement of a stranger.

Driving on roads will be impossible, without that trust. When you are drunk, and choose to drive when you judgement is poor, you are betraying the trust implicitly placed on you by every other person on the road. So isn't that something like a crime? Just thinking loudly.

I tell this as a moderate drinker myself.