The answer is simple: real estate.
When you go to Starbucks in the U.S., although the space may have places to sit, it is usually full. In other words there are no empty seats. And while the people at Starbucks will not make you leave or bring you a check so as to imply that you should leave, most people take their coffee with them, and others only stay for a short time.
In Argentina, however, coffee is an experience. You sit in an uncrowded, huge (sometimes) cafe on the corner of Santa Fe and Scalibrini Ortiz (like I did yesterday), and you can stay as long as you want. Read the paper, use wifi, chat, whatever. This prime real estate costs money as does having a waiter bring you a cup. Often little cookies and a glass of water are served with the coffee as well which also cost money.
In other words, coffee in Argentina is more of an event than a drug injection. In the U.S., you get in line, get your coffee, and get on the road (normally). Sitting and chilling out over coffee is not as common as it is in Argentina and thus the disparity in pricing.