they keep bitcoin in their wallet for long periods of time because they expect that bitcoin price in the future, it can multiply many times the current price
A big chance to double your money if you just hold it for while and waiting for a great surprise. Bitcoin price is continuously going high, we can really make money from bitcoin if you don't spend all your btc. But it is better that 50% of your bitcoin are used for investment for a chance to have a huge profit rather than holding it all waiting for a right time.