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.
People have long thought of going to work and pouring gold to save as if it was saving their money. Now still so! But young people and many people want to save both money and make more money by buying bitcoin instead of buying gold.