Scarcity is indeed a reason, but that is not the only one though. You can say that bitcoins are priced this way because people see it as something valuable and useful. Similar to gold, people value it based on how much they believe its value to be. To be honest, I would say that gold is a lot more useless than bitcoins as the latter is much much more malleable and has more real world use than gold which is just used as a jewelry or some sort of status symbol.