I strongly believe that Bitcoin early birds were very smart enough to get hold of the system smartly. Most of the dormant wallet you see holding Bitcoin for years are just alternate wallets by those who were opportuned to get hold of Bitcoin and they decided to hodle some of their wallet for a very long time while they merry with other wallets holding some worth portion of Bitcoin to sustain themselves.
However, some might be as a misplaced key phrase which the owner has long lost access to or death of the owners as already mentioned by other members here.
possible that they have various reason why they really want to hold their coins for a very long time. It's a case to case basis. There are some kind of investors who have set a proper timeline before they sell their investments, as long as they have enough money and savings for their daily needs, they don't have to move their investments unless there's a crucial reason to use it.
I read something here in forum before that one of the reasons why they haven't sell their hodling is because it's been so long, that they can't remember where their wallet IDs are, just like what you've mentioned, some of them misplaced their key phrase and Other reason is about technical issues just like Lost device and computer system issues.
It is true that most of the wallet holders might have had one issue or the other that warranted them to ignore the wallets holding their Bitcoin assets. However, that does not mean that.the issue of wallet key phrase is the only problem they are facing but rather some might be as a result of their holding for a long time so as to accumulate huge amount of profit on the long run.while some are another issue.
But in all, there must be one reason or the other that warranted wallets of that nature to be inactive irrespective of the number of years the wallet has stayed.