Basically because it keeps growing and growing like now it is at $5000 and also it is very difficult to earn but i think there should be a time when u sell it you should not never sell it because why did you have it then if you were not planning on selling it?
At some point we will need to sell in order to obtain profits it is impossible to hold forever the only scenario in which we will be able to hold forever is the scenario in which bitcoin becomes so accepted through the world that you can buy everything you want with bitcoin.