In a lot of jurisdictions, the supplier of the alcohol is at least partially liable, provided that there is some evidence that they could have foreseen a problem. For instance, it is often illegal to serve alcohol to a visibly intoxicated person, and even though that law is rarely enforced in the ordinary course of events, it does form the basis of liability for damages the drunk may cause (and can involve jail time as well as a loss of any liquor license and civil responsibility).
However, let us never absolve the drunk. Drinking is a choice*, and the consequences that follow that choice are reasonably foreseeable at the point the choice is made. Among those reasonably foreseeable consequences is being in care and control of a motor vehicle while under the influence. It doesn't take a whole lot of forethought to avoid driving, either by going out without your car or turning your keys over to somebody when you decide to take that second drink. It doesn't matter how wasted you are at the end of the night, you've had plenty of time to avoid drinking and driving long before it becomes an issue.
You can can extreme intoxication temporary insanity, at least in a clinical sense, but never forget that getting to that state of temporary insanity is almost always voluntary. (We'll take "slipping someone a mickey" out of the conversation -- that's a completely different thing.) It's not like people don't know any better; "designated driver" is not a foreign phrase, most bars will take your keys for safekeeping if you turn them in, there are taxis, the airwaves and cable have been filled with educational ads for years, etc. There are no excuses anymore.
* Yes, even in the case of alcoholism. Not drinking is a damnably difficult choice to make for an alcoholic, but it's still a choice. The various treatments for alcoholism don't give the alcoholic a choice he/she never had, they just make it easier to say no over the long haul. And in case it makes a difference, I've been saying no for a little over twenty-six years now, and haven't even had the urge to say yes for more than twenty-five years. And yes, I totalled cars and hurt people before I was forced to make the right choice.