Because ending a life and starting a life are completely different things. The opposition to the creation of a new life does not entail an opposition to the continuation of an existing life. Before someone is created there is no one. There is no one who wants to live before someone else has decided for that person that s/he would live. But as soon as someone is created there is already someone. Once someone already exists, there is someone who has interests, needs, and desires. None of this exists before someone exists, so there is no one to consider before someone is created.
Before someone is created there is no morally relevant being whose interests must be taken into account. On the other hand, when someone already exists there is a morally relevant being whose interests must be taken into account. For example, people are biologically built to survive, therefore they have a life impulse (even if their lives are really bad) and therefore, among other things, they are afraid of death. And these are already two reasons that must be considered in the context of terminating the life of someone who exists, but not in any way before someone exists, since no one has any urge to be created or any fear of death before being created.
Philosopher David Benatar argues that the standard for a life worth starting should be much higher than for a life worth continuing. And he claims that in practice, even if not consciously, this is how most people think. For example, most people think that the life of an existing person should not be terminated if a significant defect is discovered that requires the removal of an organ, but the pregnancy should be terminated if a similar defect is discovered in the fetus. Their logic is likely to be that there is a difference between starting life with a significant problem and one appearing in someone who already exists. Presumably they would wonder why anyone would start life with a problem? That is to say, most people understand and even agree that the attitude to life that has not yet begun should be different and requires a much higher standard compared to life that has already begun.
But a more interesting and important point that should be drawn from this, is that if most people agree that life should not begin with a problem, they should not allow any case of reproduction because everyone begins with a problem. No one will escape pains, frustrations, disappointments, boredom, fears, worries, anxieties, loss of loved ones, and death. All these are a necessary part of life.
If we discover in advance, in the fetus, a defect that causes the person to die at the age of 45, will we think that this is a life that is not worth starting? If so why? Because this is a short lifespan compared to the world average these days? And what if there are those who claim that in relation to the historical human average it is not particularly low and therefore it is only bad in relation to the existing situation but it is a perceptual error since people once lived less?
And among those who would argue that we shouldn’t create someone who will die at the age of 45, what will they think of death at the age of 55? And among those who think that it is appropriate to start the life of someone who will die at the age of 45, what will they say about death at the age of 35? If they argue that it is inappropriate to start a life that will end at the age of 35 but yes at 45, what about 40? Then again, if appropriate, then what about 37 and a half? And if not appropriate then what about 42.5?
Of course we don’t know about such a defect, certainly not with such a level of accuracy, and anyway the point is not to find an acceptable lifespan but to provoke thought that the person who was created will die someday, it could be at the age of 55, 45, 35, 80 or 12, but death is certain.
And the same questions can also be asked about the quality of life if it were possible to quantify it simply and know about it in advance. As with the length of life, the quality of life is also impossible to determine and is therefore the discussion is completely hypothetical, but not unnecessary, because it sharpens the issue of taking the huge risks that people take when they create a person.
For example, would you create someone who would suffer from chronic pain for a year of life? Someone who will suffer heartbreak for two years? Depression for three years? Any addiction for four years? Loneliness for five years? Losing a childhood friend at the age of 20? The loss of a parent at the age of 12? Anxiety for 10 years? Significant dissatisfaction with their appearance for 20 years? Meaninglessness for 30 years? An unrewarding and unsatisfying daily Sisyphean struggle for survival for 50 years?
These are not particularly extreme cases, these are the lives of many people, and possibly, had their fate been known in advance, many would think it inappropriate to start them although we do not necessarily think they should be stopped. And the logic is, again, why make someone start a life when it is known in advance about the significant problems they will experience during it. It’s just that this is a question we ask everyone who is in favor of creating people. Why start the life of someone who is known in advance to experience significant problems during it? Not necessarily these, and not necessarily these durations, but it is likely that at least one of them for a certain amount of time, and certainly others for similar amounts of time.
Most people probably don’t think of depression when they think of a birth defect, but it is very likely that a person with depression will suffer much more than someone with a birth defect. Still, most of us do not hesitate to claim that it is appropriate to terminate a life with a birth defect but not to prevent starting a life with the possibility of depression, or with the possibility of developing more serious defects than the congenital one later in life. That is, it is not the degree of the expected suffering, but rather the immediacy, and the proximity to the moment of formation that is decisive for most people. Some will argue that this is also the certainty, that is, in the case of early detection of a defect, we know for sure that someone will suffer from something, and in the case that no defect is detected, there is no certainty. But as we tried to outline before, there are certainties that there will be severe harms, and there is a probability that they will be more serious than suffering from a birth defect. Still, most people are adamant about terminating a pregnancy with a birth defect, and are adamant about starting and continuing a pregnancy when there is no identification of a birth defect but there is certainty of other serious harms.
But we are making the same principled claim that many people make when they wonder why someone would start life with a significant harm, and that is why make someone start a life that will certainly contain significant harms, when it is really not necessary, or desirable, or needed by whoever will be created?
The whole difference is that in cases where people agree that someone’s life should not be started, they see the harm in advance and in all other cases they do not. But they know that harms will come. They don’t know how, how many, which, when, and how much it will make the created person miserable, but they know it will happen. They just choose to ignore them.
But more important than the difference between a life that is worthy to continue and a life that is worthy to start, there is a dramatic and critical difference between calling people not to create other people, that is, to avoid actions that will dramatically affect the lives of others, and calling people to stop their own lives, that is, to do actions that will dramatically affect their own lives.
Those who live and are satisfied with their lives and do not want to stop them, may do so. On the other hand, no one is allowed to make decisions for others, certainly when it is not necessary and when it is impossible to get their consent for it.
Like everyone who was created, you too did not ask to be created, you did not want to be created before you were created, and you did not agree to be created before you were created. But it is legitimate that even though it is imposed on you, you want, prefer or are curious to continue your own existence. What is not legitimate is taking the risk that the people you will produce will feel differently.
The vast majority of anti-natalists do support the right of people to die, but beyond that, anti-natalism is not a philosophy that advocates suicide. In fact, it is anti-natalism’s opponents who tend to advise anti-natalists to commit suicide because they supposedly think life is so bad. But as explained in the answer regarding this “recommendation”, anti-natalism is not a philosophy that holds that life is necessarily bad, and it is certainly not a philosophy that encourages suicide. Anti-natalism is a philosophy that opposes the creation of new life, not the termination of existing life. The problem is with creating new people since it is a decision that people are making for others.
Those who already exist and are satisfied with their lives, are welcome to continue them. The problem is with creating others without any prior expression of desire on their part to be created, without their prior consent, and without any way to ensure that they will be satisfied with their lives.
It is your choice to continue living even though life is full of risks. The problem starts when you decide to start a life full of risks for others.