Because you don’t. Not really. There is no way for parents to raise their children without harming them somehow and without them being harmed by other things somehow. Even diligent and thorough preliminary learning, a huge investment in children when they already exist, patience and endless giving, will not prevent harms from being caused to children.
It is not clear exactly what people mean when they make such claims, it is very possible that they are referring mainly to material means, but this is the bare minimum and certainly very far from satisfying the multitude of needs at every stage of life. And if they’re referring to mental and emotional means, then no one has the necessary tools to ensure happiness for their children, for the simple reason that no one knows what is really needed at any given moment in order to ensure it.
And even if you specifically had all the necessary tools in all possible respects (even though this point is hypothetical it is difficult for us to even write it seriously because the spectrum is so broad that it is clear that no one can meet such a criterion), the rest of the world does not. There are so many ways to get hurt out there, that no one without exception including very talented and devoted people can constantly protect their children from any potential harm. And there are real dangers out there that can scar people for life if not destroy them. The intuitive thought is perhaps atrocities such as kidnapping, sexual exploitation, murder or abuse. But even much less unusual and very common cases may harm people for life.
The world is organized and managed in such a way that no one can really be happy. If there is a way, it is probably only a total disconnection and a thorough purging of every will, desire and cause of disturbance. But no one wants to have children in order for them to be separated from him/her. Besides, you have to stop for a moment and think about it, what does it mean about the world and life that only an almost total detachment from it can bring happiness? What does it say about the world if only those who manage to neutralize reality and enter a completely disconnected state of consciousness where they don’t really feel anything, don’t want anything, don’t regret anything, don’t expect anything, but are simply there, only then can you start talking in terms of happiness? This is an indictment for the world, and even more so for those who insist on throwing more and more people into it.
And this disconnection, in addition to the fact that parents do not want children in this situation, is also a terrible testimony about life because someone who can be happy in such an ugly world cannot really be a beautiful person. You have to be super egocentric in order to ignore all the horrors, turn inward and feel peace despite everything.
But even if the chances of happiness were very high, the mere fact that there is a risk of an unhappy life is a sufficient reason to avoid reproduction.
Because no one needs, wants, or must exist before being created, because no one has an interest in existing before existence is forced upon them, because no one is in another world hoping that they will finally be brought into this world, because there is no other world outside of existence, and there is no other state of existence outside of life, no one waits, needs or wants to exist before existence is forced upon them, it is sufficient that there is a potential danger of unhappiness in order to avoid the creation of people. All of this is important in general, is very important in relation to reproduction, and in particular in the context of this specific excuse because since that is the face of things, everyone who exists was created because someone else wanted it to happen, because it was in someone else’s interest, maybe many others, but necessarily not of the one who was created because they did not exist before they were forced to exist. So basically reproduction is people deciding to take a risk on someone else’s life. People want to create someone who, at least before being created, did not want to be created for the simple reason that they did not exist and therefore was not in a position to want or not want anything, and there is a risk that this someone will be unhappy. So the breeders are taking a risk on someone else’s life. Statistics are not a decisive factor in this matter. That is, of course it matters a lot what someone’s chance of being unhappy is, but it is not a decisive factor because there is no way to know what someone’s chance of being happy is, and because it is sufficient that there is some chance of unhappiness for reproduction to be morally wrong. And again it is important to mention that it is not in the interest of the created person to be created. If it were, only then it would be possible to weigh the possibility of unhappiness and happiness against the interest of people to be created. But since there is no such interest, and since there is no harm to anyone who was not created, no one is deprived or lacking, it is a risk at the expense of others without any justification.
Even if the great majority of people were happy and only a small minority were unhappy, since no one would be harmed or lack anything if they were not created, there is no harm in preventing the creation of happy people. But it is clear that there is harm in the creation of unhappy people. It is therefore clear that it is better to avoid creating people. This guarantees the prevention of all the harms of anyone who would have been created, and it cannot harm someone who was not created because someone who was not created cannot be harmed or feel deprived or unfulfilled. S/he who is not created does not exist in any way, therefore s/he can’t be deprived of anything. There is nothing wrong with preventing positive experiences that no one lacks. On the other hand, having negative experiences is by definition a bad thing.
No one has the means and ability to ensure that their children will be happy. On the other hand, everyone has the means and the ability to ensure that their children will not be unhappy by not creating them.
There are no means to guarantee happy children. On the other hand, there are means to prevent unhappy children – contraception.