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.
Indeed. The main reason why they hold gold and bitcoin is that it has limited supply and the income you could get if you hold. Furthermore, holding are one of the best way to earn profit. Just hold your bitcoin and gold and wait for the right time to sell it.