That's because it's one of the conditions under which the license is granted. Driving is a privilege, not a right. By accepting the license you accept the terms of driving, one of which is you cannot legally drive while intoxicated.
No. I don't see driving as a privilege, since a vehicle is YOUR property. The reason that drunk driving isn't allowed is because your inebreation is likely to harm others. To a lesser degree, your innebreation from being high is also likely to harm others. That's why the great majority of Americans stand against drug legalization. It really is about protecting the general welfare rather than I want to control what you do, as you see it. I understand your position, but in the end there is more to it than that.