Carl Marx himself was of a privileged class. Your average Starbucks socialist probably doesn't put much thought into the practical ramifications of their ideology. Maybe it starts as a form of teenage rebellion. As long as their ideology is rejected by society at large they don't have to outgrow it. It gives them an excuse to complain and act like they're more enlightened than everyone else in the capitalist society that they rely on. Kind of like people who call themselves vegetarian but still eat fish and chicken that was slaughtered by others.