Because investors who don't like to sell or who want to continue accumulating bitcoins are people who haven't set a plan to sell they will be more likely to continue buying as they like.
Every investment should have an end, just as every chapter ends. If the investment doesn't end. There is no point in investing, I would consider it worthless. You have been investing all your life, but have not been able to enjoy the money invested. I consider such an investment futile. But I am not saying that you should end the investment too soon. You invest a certain amount and after getting your desired profit exit the investment and enjoy that money and start investing again. You can continue like this as long as you are able to generate income. Build a sizable investment fund years before you retire from your career so you can enjoy it in retirement. If you only invest all your life, that money may one day be left uninherited. Because human death comes unexpectedly.
Many Bitcoin investor I know want their Bitcoin investment to continue even after them. I'm one of such people because I have decided that Bitcoin is part of the legacy I will pass on to my children. If you plan to end your investment one day, it means you do not believe in continuity and that is a very bad situation to be in. Even in other business endeavors, it is always the joy of parents that their children should be able to continue their legacy that is why you see some families known for certain line of business for several generations. Such is possible with Bitcoin and many options have already been suggested on the best way to pass on Bitcoin to heirs.
I real long term HODLer of Bitcoin must start making plans of the inheritance of their Bitcoin portfolio even though they are not expecting to die any time soon. Such plans will have a way of preventing any eventuality. I know that some of the lost Bitcoins might be traceable to death of their owners without plan in place to pass it on to others of trust and importance.
Ending a chapter of an investment that is profitable and has been beneficial is a wrong idea, Have you imagined why companies, institutions and investments are past down to generation after generations. Because a profitable investment is like holding onto a treasure that should be adored and kept for long as possible. Holding onto a successful investment that continues to generate profits is very very necessary. A premature closure of a profitable investment can be a missed opportunity as it can still generat wealth and growth. Unless the investment is no longer bringing profits.
Bitcoin has been profitable and still shows a lot of potentials, there is no need to close the chapter or sell all your Bitcoin holdings because you have made profit from it. You can take profits from your bitcoin investment when it has yielded while still maintaining your investments, this way you can secure returns and hold onto a potential assets (Bitcoin). Before closing or ending any chapter of an investment you have to evaluate whether it still generating profit or not, does it still have potentials or not.