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.
I think people may not want bitcoin in the future is that because they dont know the real purpose and value of it. And also maybe they dont know on how to have it unlike gold easy to get when you know where to find it and the risk of being scammed and lost is less than bitcoin. But if they know what bitcoin really is. I believe they going to embrase it.