"You don't have to go home but you can't stay here!"

Sitting in a car while drunk is a DUI in many if not most states.

Not really. You can get a DUI for sleeping in the backseat of a car and you can get charged with vagrancy or public intox for sleeping it off in public. You have to go home (or to someone else's home), and the car is probably the only way to get there.

That's why designated drivers and taxis/ubers are so important. The only alternatives are dangerous or illegal unless you can find and afford a spontaneous hotel or you live in a big enough city to have useful public transit.

It seems to me that there's some weird propaganda going on in this thread. I've only ever known one person who has driven while drunk and the only reason I know that is that he confessed it out of deep shame.

I guess it's possible that everyone I know is out joyriding every time they drink and just not telling me, but assuming that's not true, I - being from and in the US - never really experienced a culture remotely supportive of drunk driving.