Because they will be different people. You can’t make projections from your life to other people’s lives even if those other people are your children. Everything in their life will be different from your life – the genetic load, the womb environment, the initial formative experiences, the experiences later in life, the education, the environment, the society, everything will be different. Other people’s lives will not necessarily be similar to yours just because they will live near you. Are your lives and those of your family members so similar? Is your life so similar to the life of your parents?
Most people tend to think that bad things only happen to others and will never happen to them, certainly not when their lives are generally good in their eyes. But terrible things necessarily happen to someone, and there’s no reason why that someone wouldn’t be your child. It is very likely that many of the people who have had terrible things happen to them in life are the children of people who thought their lives were good and so there is no reason that their children’s lives wouldn’t be good as well. That is until they realized that there is a reason and that is that terrible things happen to people. And in many cases without those people doing anything to summon those terrible things, and often without them being able to anticipate them in advance.
And if the happiness of other people is important to you, why create new people, people who will take more resources at the expense of others, who did not give their consent to their creation, and will be at constant risk throughout their lives for serious injuries, instead of devoting all the resources including all the time, energy and money that you planned to invest in raising your children, to people or creatures of other species that already exist because others created them, and need assistance? You can make others happy and without all the vulnerabilities and difficult problems that accompany the creation of new people. That’s double profit. You didn’t create a new problem and you helped many others who are already here and have problems. How does that not make more sense?