I believe that vast numbers of Bitcoins were found and lost in the early days, because back then, people were mining just because Bitcoin was an interesting technology project. When they lost interest and quit mining, wallets were easily lost in OS reinstalls etc. because why would you bother backing up a bunch of funnymoney?

So I predict that a big portion of those early Bitcoins that have not been moved, will never ever be moved, because the wallets are just lost.