Either they are alcoholics who can't control themselves or simply just think they are still under control of their ability to drive despite being impaired. Many people just don't know what 0.08 BAC feels like. In college, I got the opportunity to blow on a breathalyzer (not because I was arrested) and found that despite not feeling drunk, I was over the 0.08 limit.

Addiction, mental illness, a defacto requirement to drive to get around low-density towns where walking is often extremely dangerous due to lack of sidewalks and fast roads.

Alcohol abuse has been around about as long as we've been human. We've just constructed a society where Alcohol abuse is far more likely to pick up collateral damage.

There are no beds in most bars and nightclubs.

Alcohol inhibits people's decision making skills

Because they are drunk and want to get home.

Or because they are drunk and want to go somewhere.

That's all there is to it.