I suspect the reason is a pretty banal one, springing from the same sentiment as when young people say they don’t care to reach old age, it’s all over after you reach 30, 40, 60 take your pick. A simple lack of imagination. Then they reach the target date, and surprise, surprise, no way do they feel like dying, or even old. Like in that quip about “old is always five years older than me”.
It’s the same superstition about “don’t interfere with nature”, where “interfere” is always a shifting target depending on what is attainable at the moment. Let nature take it’s course, but when a drug, convenience or medical intervention allows a previously unattainable cure, suddenly the “nature” proponent is more than happy to take advantage of it, nature be damned.
The naturalist fallacy folks are singularly lacking imagination, we can’t live too long because of overcrowding (duh, as lifespan increases, people simply have fewer kids), we don’t want to spend decades getting increasingly frail (that’s not how it works, you can’t be young until 30, then spend the rest of, say 150 years getting ever more frail), we would all lose our friends (don’t bogart it all, share the cure!).
I especially like the “we’ll get bored, not know what to do”. If so lacking in imagination, curiosity, thirst for knowledge, distant horizons, exploration, experiences, appreciation, I recommend you kill yourself, not because you are bored, but because you don’t deserve a long life that would be wasted on you, don’t waste resources, leave them to those who are appreciative of the incredible miracle of the gift of life in a cold universe, the greatest miracle of all miracles, the insane wonder of LIFE. Leave life to those of us who would be grateful for the greatest gift of all: more time on this planet… and perhaps other planets. Why are ye fearful, ye of little faith? Little faith in life and its beauty, the possibilities of the human intellect, science and an incredible future? If you fear the future, you can always cut it off right now. For the rest of us, we walk toward the light, with optimism, faith in humanity and appreciation of beauty and the endless opportunities of an eternal universe.