"Why haven't the earliest participants sold their Bitcoin, and some haven't even touched the coins in their addresses, considering the high price of Bitcoin?"
Why those OG's didnt sold their bitcoin? Case to case basis
If someone do still have access to their coins then its simply;
They do tend to hold forever whether for Profit reason or for its utility or showing the support
Target price hadnt met yet
They dont need money yet?

For those dormant accounts or doesnt have any activity since.
Lost keys?
Dead owner
Totally forgotten that they have Bitcoin
If i were you, then it would be just that better if you dont mind much about those non touched coins or wallets.
We dont know on what happened into their respective owners. One things for sure though that if the time comes that they would
be having access or getting in touch with their coins then its neither they would be selling off for profits likely.