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.
We all know that both of them have limited supply so that once one of them rich the limited supply the price will totally become expensive. I think the real reason why they hold their bitcoin is the income that they could get if they hold it for many years. We all know that holding is one of the key to have better income so that you should wait, do not panic, just hold it to gain much money.