My sons didn't bring dates, but pretty much all of their friends went, guys and girls, just in a gang (like they tend to do - run in packs).
Anyway, like Shirley said, the builds are incredible for Xmas formals (they want to show it off), and they are not like a formal where everyone has a date. The way it works is the fraternity guys usually go with their dates in large groups (like by pledge class) to a swanky restaurant downtown, (for some events like parent's weekend or spring formals the parents are invited and go too and of course get hit up for the dinner tabs
) then get to the house around 11:30. The hs kids just meet up at the houses, and will go from one formal to the other.
It is used as a recruiting event, therefore it is free - dues pay for everything, that, along with social dues. If you knew what the recruiting budget is for these houses, you wouldn't be surprised that they foot the bill for recruits.
As for the drinking - all the houses have security (at least my sons' fraternity) does, and they also hire contract bartenders to dispense the alchohol. This is to try to combat the underage drinking, and to limit liability, but how well it works, I don't know. When we've been to events, I've seen the bartenders check Id's but I've seen kids I know are not 21 with a drink in hand. so....