It's just part of the social standard that many cultures have had for generations. Many of the earliest public establishments for people to gather and mingle were pubs (or the equivalent). It became part of the whole deal.
Sort of the same for "Why do you get popcorn when you go to the movies? What's so 'popcorny' about being in a theatre? " Some people only eat popcorn when they are at a movie, and some always get popcorn when they go to a movie. Why? Because it's just part of the ingrained structure of the social activity.
For a loooooong time, most public parties/gatherings, involve alcohol to some degree or another, and thus it's...well I don't want to say expected, but it's certainly assumed by many people that booze will be included in most social events.
Of course that expectation is slowly changing, as more people are of adult age that don't drink alcohol at all (like myself), so it's not so much in demand. I think it's a mix of just simple choice to not have it at your party, because the majority of your friends don't drink, so why spend money on booze? I know if I hosted a party, and invited my friends, maybe 1 person would drink. And by that I mean they are someone who would consume booze, but still probably wouldn't because they just don't bother much. The rest of my friends are almost 100% non-alcoholic in nature. It's just not part of our social structure.
I also think it's partly a reaction to people wanting to prevent any "WHOOOOOOOOOOOOO!" *tear off shirt and do stupid stuff* type behaviour at their party. That stuff can pop up very unexpectedly when you add booze to the equation. So, they might just decide "Meh, no booze, that way we'll likely reduce the chance of Asshatery to 0%"
Sort of the same for "Why do you get popcorn when you go to the movies? What's so 'popcorny' about being in a theatre? " Some people only eat popcorn when they are at a movie, and some always get popcorn when they go to a movie. Why? Because it's just part of the ingrained structure of the social activity.
For a loooooong time, most public parties/gatherings, involve alcohol to some degree or another, and thus it's...well I don't want to say expected, but it's certainly assumed by many people that booze will be included in most social events.
Of course that expectation is slowly changing, as more people are of adult age that don't drink alcohol at all (like myself), so it's not so much in demand. I think it's a mix of just simple choice to not have it at your party, because the majority of your friends don't drink, so why spend money on booze? I know if I hosted a party, and invited my friends, maybe 1 person would drink. And by that I mean they are someone who would consume booze, but still probably wouldn't because they just don't bother much. The rest of my friends are almost 100% non-alcoholic in nature. It's just not part of our social structure.
I also think it's partly a reaction to people wanting to prevent any "WHOOOOOOOOOOOOO!" *tear off shirt and do stupid stuff* type behaviour at their party. That stuff can pop up very unexpectedly when you add booze to the equation. So, they might just decide "Meh, no booze, that way we'll likely reduce the chance of Asshatery to 0%"