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.
yes why do we hold bitcoin and gold, yes you were correct they are both limitted supply and loved by people. asight from it both increase its value continouosly. specially gold it increase its value alobg the century. and its supply is very limited because mining is really prohibited because it affect the environment and the resources.