Our claim is not that it is better not to be created in the sense that non-existence is better. The reason we claim that existence is bad is because we will have bad experiences in it, not because non-existence is good, or better.
In the same vein, it can be argued against us that, at least for certain people, existence is good and not because it is better than non-existence, but simply because we have good experiences in it. We do not deny that this is a possibility, but every existence is necessarily accompanied by vulnerability, and on the other hand, no one experiences the loss of good experiences or the prevention of a good life if they never existed. There is no reason for anyone to have negative experiences even if we are sure that the positive ones will surpass them, because if s/he did not exist s/he would not be harmed by the negative ones nor by the absence of the positive ones. But more importantly, while if we avoid creating someone who will live a good life we have not harmed that person in any way because those who do not exist do not experience missed existence, if we create someone who will live an unhappy life we have certainly harmed that someone because those who exist are definitely harmed by an unhappy existence. And since we never know ahead of time what kind of life the person we will create will live, and it is never in our power to prevent someone from an unhappy life since a multitude of harms can befall people at any given moment and a multitude of factors are beyond our control, it is always morally wrong to create new people.
So our positions don’t stem from the idea that it is necessarily better for someone not to be created, but rather from the fact that it is necessarily morally wrong to create someone. Every life of every person no matter what their status is at a given moment, was created without consent, can turn into hell at any given moment as a result of some event that will happen without warning, is full of unavoidable harms to others, was not created for the created person or because it was in that person’s interest but necessarily as a means for the ends of others, exists in an unfair world, is basically purposeless, and meaningless in itself, and therefore the created person will have to find some kind of meaning on their own. No one is really happy and many people are very frustrated, no one really lives the life they would like if they could choose or the life they hoped to live as an adult when they were children, no one chooses in advance even one critical characteristic about themselves including their genetic load, their womb environment, their nutrition, influential stimuli as a fetus, their family, their immediate environment, the time of their existence, the society they were born into, their country, their personality traits, formative initial experiences, etc., and no one can really influence any of these, that is, all people are condemned to certain conditions of formation that were chosen for them and they are stuck with them for the rest of their lives when the most they can do is work very hard in order to change them, probably to a very small degree, and even that’s within very narrow limits only. All this and more is always wrong, and regardless of how someone is satisfied with her/his life. We are anti-natalists first and foremost because reproduction is always morally wrong for all of the above reasons and many others, and not necessarily because the life of the person who was created is always bad or because it would always be better for everyone who was created not to be created.
The moral basis of our anti-natalism is not derived from the claim that it is always better not to be created, or only from the degree of well-being of everyone who is created, but from the risk of unhappiness of those who are created, the lack of consent to be created, the coercion of the created, becoming a means to the ends of others, the purposelessness of life, the injustice of the world, and no less important than all of these, and perhaps even more, the inability of the created to avoid harming others.
Not being created is indeed not a better situation for someone, and it is not even necessarily always bad to be created, but it is necessarily always bad to create someone.