We can speculate about their reasons for not doing so, but their reasons do not really concerns us and are theirs alone, however if you want to speculate why this happens we could imagine many reasons about why this is the case, like their coins being lost or not remembering they have those coins at all, after all would you remember where you placed a dollar more than a decade ago? Also it is possible their holdings are so valuable they have in fact sold a fraction of them while the rest appear to not move, but the owners of those coins are just waiting for the right moment to sell some of it again.