"More financially injurious, however, is the exorbitant rents that millennials often pay..."
House parties take space. Space costs money, especially in and near Manhattan. They also require either lots of room between homes (to keep noise down) or amazing soundproofing. Neither of which is common in Manhattan.
And everyone pays crazy rent. Remember when housing prices crashed in 2007-2008 and the government was brainstorming ways to keep them up? This isn't a force of nature, and it's not extra hard on millenials. It's policy. If anything, it's harder on families, since they need more space for kids (who don't pay rent) and have less tolerance for walkups (strollers) or rough neighborhoods.
Besides, there are plenty of house parties across the country. They call them BBQs or pool parties, and they have them during the day in places where people with steady jobs can afford to own or rent a home with a yard or pool. You know, flyover country. The places that don't read the Style section of the NYT.