Pretty much everyone in the US owns a working fridge. If you just want to get drunk at home, you just buy a case of beer and leave it in there. It's actually much cheaper to buy beer by the case in most places
Although I also assume people are drinking these beers in the car, it does seem like a bit of a leap… I mean, an alternative is that they just figure they’d rather have the beer ready right when they get home (instead of sticking it in the fridge for a bit).
The same reasons convenience stores sell single cigarettes, and people smoke them in their cars. Compelling cravings don't care about your externalities and don't want to wait. Lots of people don't want the people they live with to know how much they actually consume. Some people live with others who don't allow beer in the fridge for various reasons. Some have a housemate who can't be trusted around a surplus. Lots of people like to think of themselves as the kind of person who doesn't need to have a whole pack around, they just want this one and maybe another one. Depending on who you're hanging out with, sometimes it's nice to go to a BYO party and not contribute a surplus if that would get abused. And as always, who said they waited to get started until they were driving home anyway?
But that requires planning ahead. The whole point of a convenience store is that it's a convenient place to get small quantities when you didn't plan ahead, and people are willing to pay a premium for that.