Because as we can see it is growing in value, so as long as you can still afford to buy bitcoin grab it, because sooner or later it will grow double of value of what we have now.
Bitcoin is a good investment right now because of its continued increase in price. The more bitcoin you have the bigger your chance to gain more profit. People with lots if bitcoin in their wallets can surely sell their bitcoin right now and got the chance to buy another bitcoin and sell it again when the price is higher.