Because the drunk driver's lack of consent isn't the problem with drunk driving: it's the lack of consent from everyone else on the road.

The issue with consent while drunk isn't about what you do to yourself while drunk: it is about hurting other people. You have the right to get drunk to the point of passing out in front of the stereo with The Cure's worst album at full blast, the dishes undone, leftovers on the stove and that still doesn't give anyone the right to steal your stereo, sharpie your face or fuck you (even if they shake you hard enough to get some vague affirmatory groan out of your body).