Why do Americans bulk buy milk and how does it not get spoiled ?

I've been watching a lot of Costco shopping mini vlogs on instagram. In a vlog someone bought 6 gallons of milk (22.712 liters) for 6 kids and 2 adults which according to them will last them for a week. Also watched a lot of reels where people bulk buy milk which will last them for 15 days to 1 month. I know that the milk is pasteurised and all, but still most of us Asians don't use a milk pack if it's been opened and lying in the fridge for more than 3 days. How do Americans do it ? Does the milk not get spoiled ?