There are various reasons
Most fall under the category of been unable to access their coins
Either through loss of password or death
While the minority are still holding.
In all I can only speculate since i am not one them.
Yeah, you said pretty much everything that needs to be said.
They may be aiming for more profit. It's possible that the early investors are not selling their Bitcoin even if the price is high because they have plans on using their investment for their retirement or they have their own plans for their own future or for their family.
...but one reason why OG's are
not hoarding Bitcoin and not selling it is for making a profit. Each halving was an opportunity to sell the stash and secure a nice fortune for themselves - not saying there are no OGs who are like this but it is highly unlikely that this not what most of them are doing. On the contrary, this sounds like more like something the newcomers would do.