It’s maybe a cultural thing beyond startups too, merely that the demographic that is most startup-y is also most drinking prone.
Community / social events? Meet at the pub!
Entrepreneur mixer? Meet at the pub!
Design conference? There’s an open bar and half of your ticket price goes to supporting that, even if you don’t want it.
Those monthly social / mobile / Wordpress / design etc etc meets? In a conference room, in the back of a local bar.
Heck, with exception of the hiking and biking one, all of my meetup.com communities drink as part of the activity. Board games? Why not board games in a bar! And so on.
So for me it’s like, if you want to have any sort of social interaction in life you better get used to paying $4 per small glass of orange juice.