For one, I'm not even sure I could get a cab to come out to any bar near me, or to my house. I probably could, but I feel certain that it would be extremely expensive to do so. If you're not quite so rural as me, but not so urban as to have a mature metro system, then it's my understanding that it is a fairly constant problem for some people that there are areas of town that cabs refuse to pick up from and/or drop off at.
Then there's the logistical problems. If you are going out with friends, you may not know beforehand that you will be drinking. That means you may well drive your car (while perfectly sober) to a restaurant/bar or similar place. When you get out, and realize that you're impaired, the first problem is that if you call a cab, that means you're leaving your car overnight. Some places, that means it may be towed (because they don't permit overnight parking), other places it might have a higher chance of being burgled.
Then there's the additional problem that if you leave your car, then you're going to have to come back and get it, which means another cab trip the next day, and some extra time from the next day. That basically means that you can't choose to drink at all on a day that's before a day you have to be somewhere in the morning (like work, or taking kids to school, or what-have-you.)
None of these are insurmountable obstacles, but they are obstacles, and they are annoying, and if you have a self-driving car, they all pretty much go away (so long as the law allows you to ride in a self-driving car while drunk)