There's really mainly only 1 Reason or a Group of Reasons under the Core Reason Why People Detest and Repudiate Anti-Natalism

And when one things very rationally about this, and avoids anything to do with the ego or being childish, particularly about one's own existence or place and benefits in this world, it's just much more freeing to admit the truth about it.

But the main reason people hate Anti-Natalism so much, is because it is a negatively-oriented idea.

It's something based on inaction versus action.

Anti-Natalism forces something very uncomfortable for most people, which is a deep form of self-reflection about very uncomfortable truths, stark realities, including the inevitable mortality and facing the fact that there is no true purpose in this world, or the fact that things could be [and yet are] much darker and sinister than they ever thought.

This essentially causes a "violent backlash" in many people, especially NPC's, over the idea.

The other issue is a combination between the Linear thinking processes of Humans (only in the moment),  and the fact that they are forced to contend with the fact that they were yet born here, and many years they have lived or invested into this world, and have to have something to believe in or pursue while they try to make the most of their existence in this world.

Otherwise, they become depressed, even violent or unstable in dealing with the thoughts surrounding their existence and why they are here, and it's too much for them to handle, so they rather buy into the illusions, or even religions, which are why religion is so popular for many.

Religion puts a part of their brain "asleep" where they can still have a bogus form of optimism and hope, because deep down they know this place is hell, but they don't want to admit that.

Religion makes people believe that having children is a "blessing" from God.

Religion makes them believe there is a purpose here and that this place plays an important role in development or ascension, even though we all will leave this world without any Family we create, without the love relationship we had (in most cases), without any of the money or possessions or any achievements we created here.

And Humans spend a lifetime obsessing over these things, which will yet not serve them in the future, not even in a future reincarnation.

It's too scary of a thought to reject the status quo, especially since the majority of people buy into this world and invest in the world's problems, business cycles, growth, war or other things we are faced with, and so there is this compulsion to have more children thinking that it's somehow the solution for this world to help "fix" or "heal" it, even though it's more equivalent to adding more wood or gasoline to the already burning fire.

Perhaps the most uncomfortable revelation to most Human Beings, is that if you listen to and weigh in their grievances with this world, or the challenges they wish to tackle, say even something as trivial as creating a perfected "Universal Healthcare System", the fact is that when they are confronted with the reality that the only reason they care so much about these things or having future factory workers or people to pass the torch onto for their own tribe to "survive", doesn't actually come from a rational or significant place.

It's from own inner ego, and all because they know nothing else but this world / existence.

If they knew of a better existence and could view Life on Earth like one would view the outside world through their window blinds, they would begin to scoff and find themselves revolted at how much they became so obsessed with this world or even put any energy and constant efforts towards it, seeing how much of a waste everything really was, and they would think to themselves what a clown they must be (or look like) to Life forms or living in a much more ideal reality than this one.