there are many reasons why they don't sell their bitcoins, the most common reason is that they lose their phase .  they think btc is worthless so they forget it  and they realize when btc is as valuable as it is now and they forget where to save their seeds
one thing is impossible if BTC owners never sell their BTC especially since they have already made a 1000x profit from when BTC was first introduced.

I hope the person you mean is not Satoshi Nakamoto, because he will never touch the Bitcoin in his wallet ever. Different cases may happen to the people you mean, many events that were never expected before can make them no longer able to touch their Bitcoins. Common cases occur due to loss of access to open the wallet and death of the original owner.
If the wallet with Bitcoin stored in it can no longer be accessed by anyone, then the circulation of Bitcoin will decrease. It is necessary to educate traders or investors who are just starting to invest in Bitcoin to always protect or store their private keys in a safe place, because if they lose access to open their wallet, their assets will be frozen forever in that wallet.

Not everyone are in the side of selling their bitcoin or have that passion of selling their holding due to either wanting to take profit by selling their total holdings while there are other who are sole traders and they sells whenever they noticed every little changes in the market and would want to utilize every given opportunities to increase their profitability. Those wallet with thousand of bitcoin in them that hasn't made a transaction for long time could be as a result of misplacement of keys hardware wallet, lot of people faces this challenges because of war or flood victims especially fire incident can likely cause wallet to be entirely dead and not being active.

Well, we are all speculating but I believe that we can't have another bitcoin out of this bitcoin we already had and again Satoshi won't come revealing themselves. Who knows if they are company or groups of persons that comes together to create bitcoin, so in my opinion we can have only central bitcoin though any other coin can be created but not as exactly as Bitcoin because bitcoin has already taken the place as the number one currency that can never be beaten by another coin out there.

You didn't missed anything single moment, you can still buy bitcoin hold as little as you can, maybe who knows what tomorrow would give birth to?
Tomorrow's we can find Bitcoin worth a half million or almost a million and the fraction you managed to accumulate today will worths hundred of thousands tomorrow, so take the bold step and start your bitcoin journey.

Most the time people forget their password so their tokens remains saved in wallet forever so they cannot use it neither for profit nor for uses. BUT their is also another reason for this like some people have set a goal due to which they are unable to sell their coins before they reaching to that price. There are different reasons but it will depends on a person's mind that why he is not using his saved coins. In my opinion people often want to wait for four years of duration and sell their coins only at the time of Bull market. Some people are too rich that they don't require selling of their coins so that's why they wants to hold bitcoin for years.

I think many of them didn’t backup their private key and have lost access to their wallet. People were careless with wallet security back then, the value of bitcoin was not high so it wasn’t really a big deal to lose sleep over. Once in a while, we see a dormant address suddenly become active and moves coins out the wallet. Another reason would be death, the owner of the wallet may have died and did not make provisions for his family to inherit the bitcoins.

Cause the market's looking promising. We got the halving at the cusp of happening next year, some serious uptrends in bitcoin and other cryptocurrencies with no real signs of ever slowing down, and a pretty thriving crypto industry that's continuously expanding. So it just makes sense for newbies and older participants in this industry to not sell their cryptocurrencies this early.

Although if you're talking about bitcoin's earliest adopters, beats me really. For the most part some of them have ludicrous amounts of bitcoin that they can't just cash out on a whim or it would cause panic buy on everyone below the ladder. So they time their buys, slow their transaction processes, and make moves that don't startle the market so as to not risk devaluing their cryptocurrencies.

I disagree as anyone owning BTC has a vested interest in those coins.

One) they could be consider abandoned and confiscated.
Two) If we wake up tomorrow and 1,000,00 are liquidated over night btc would crash like mad.
Three) everyone needs to realize the power those 1,000,000 plus coins 50 in 20,000 blocks or so wield over btc in general.

Do you care about that? There are many reasons behind it and you never know the truth, so it's none of your business.

You could say this is the right reason.
We heard some news from them losing bitcoins because they lost their initial phrases, passwords, dead people that are mentioned does make sense but indeed we all just speculate on their bitcoins at the beginning not moving maybe because of the above reasons mentioned.

Hodling bitcoin for 5 to 10 years is not easy as it is. Those who have succeeded hodling this long might have very deep faith on bitcoin, and luckily their fiat funds are stable and sufficient enough that they won't even think and get tempted to sell even a portion of bitcoin. While others have still faith on bitcoin but due to emergency events that rise all of sudden, no matter how much they want to prolong hodling, they have no choice but to forcely sell their coins early.

Bitcoin gets even more valuable and highly expensive every year. That's why some have chose to hold for their future retirement, while some bitcoin have no trace of transactions probably because the owner has forgotten it or has definitely lost his seed phrase while its still worthless at the time he first purchased it.