UHT milk is just not common in the US. It's not rare, but you won't see it in very many homes. The milk in paper cartons that we in the US speak of is almost always pasteurized whole or skim (2% or 1%) refrigerated milk that is good for about a week max after opening, and not a whole lot longer if refrigerated unopened. I kind of like the convenience of UHT milk (the kind you can store at room temp for a good while if unopened) and to me, it tastes less "cowy". I don't like a "cowy" taste, but then I'm not a milk drinker. Milk in plastic cartons is most common, but there's plenty milk in paper cartons alongside those plastic containers in the refrigerated section most US markets.
Google cone-shaped milk cartons. Interesting bit of historical trivia.