Come on - how long does it take to train a person to make a cup of coffee?! Are you really going to tell me that making my girlfriend's overpriced strawberry iced mochichina or steamed whatever takes anything more then 15 minutes of training and using a blender?
Actually, yes. I've dated a barrista, and had a few friends who were, and I can tell you that there's a lot to know. First off, even if we followed your assumption of 15 minutes (which is wrong), we'd still have to multiply that by the 10 or so drinks that any barrista would have to know to make. Then we'd have to account for the fact that there's a physical technique to it. You see, it's not a matter of simply pouring together ingredients, but of doing it in a quite specific way, which requires experience and practice.
Admittedly, it's probably learnable in a matter of weeks. But, that still requires the person to actually care, and to be tought by someone who does as well. And I doubt that's altogether common. Especially if the clientel is under the illusion that it's an unskilled job, and treats them accordingly.