Let's talk about the time that Warren Buffet bought about 129 million oz of silver at $3.50 to $4/oz. around 2002-2003. Then he dumped all the silver in 2005 around $6/oz. Good timing on a great trade, right?
By 2007 silver was up to $17/oz.
By 2011 silver was up to $48/oz.
Furthermore, many of these billionaire's fortunes are not exactly liquid (at least to them.) Much of their fortune is the value of the stocks that they hold for their companies. Why would they liquidate a substantial portion of their interest in their own companies? I'm sure many of them hold their own companies very near and dear to their hearts. Just like Satosi has HODL the vast majority of his bitcoins. Why risk hurting your baby by crashing its market, by liquidating even 5% of your holdings?
what about because humans needs to eat at least?
if everybody will only HODL there will be no trade no volumes nothing, so thankfully there are people which are not HODLING till they die....