The [rural] US is much, much more spread out than most countries. This means that people in the rural US think of driving as a necessary and basic component of life; driving is just "how you get places", if there are places you need to get. So drinking and driving, though shameful, is necessarily normalized (not wholly, but somewhat) by people who are drunk, and yet need to go places.

That, and by the fact that in a lot of the very rural areas, drunk driving has a rather low fatality rate, given that there are no other cars on the road and no obstacles to crash into. If you run off the road, you just drive into some corn or wheat or alfalfa, or at worst hit somebody's cows. (These are the same places you'll see people e.g. allowing their young children to drive trucks or motorcycles—even by themselves, at night. There's just nothing much that can go wrong.)

(I think this is all also true of rural Russia, which is a large part of where the American stereotype of Russians as constantly drinking comes from.)