The real reason is that they have limited supply, and people want them. (people may not want bitcoin in the future btw) People always want gold and they want to gather as much as possible in their lifetime. It is the only solid asset which holds its true value through the centuries.
One thing I learned is hold something people want specially those of value and limited resources. Rich persons and tycoons tend to monopolize these things such as oil and gold but now that bitcoin has come to stage they hardly able to monopolized it since it takes them by surprise and at the same time can be earned by everyone regardless of their status and profession.