I am a little older, so maybe I have missed some of what you are seeing if the startups you have worked with are full of 20-somethings. The startups I have worked with — and started myself — have been free of more than occassional dinners out with drinks offered, but not required. There are always non-drinkers in the group at these events. If they are serving a white wine with dinner, I will drink water too, thank you very much. And yes, in one of the companies we would make a monthly trip to a pub where they serve different micro-brews every time we were there, but they also have awesome food and I never saw anyone from our group leave drunk. Among the people I have worked with, it is bigger shame to get drunk than it is to be a non-drinker. Perhaps I have just been fortunate to work with some very professional people who released their stress with a workout or a weekend hike: I am not sure.
Within the larger companies I have worked with, there were a very few individuals who had a drinking problem, but it was still more than what I have run into at startups.
The shared office space where I am now has an open tap in the kitchen, but it doesn’t seem to get much use except on Thursday nights, which is the beer, wine and popcorn night. On Thursdays, I have never seen anyone hanging around for more than an hour after work and no one gets drunk, even though we have a fair number of 20-somethings in the hive. I guess they are afraid of the evil eye they will get from the rest of us if they tried something stupid.