If you had a prisoner's dilemma game where people were trading and anyone could create currency then all would defect and create currency. By making substantial expenditure of energy the cost of defecting, the game loses its prisoner's dilemma quality. This is what makes bitcoin unique IMHO. Other systems have no structured way to create currency that doesn't rely on a particular party not defecting. There's tit-for-tat, but increasing the price to defect works so much more neatly.