It's simply a better state of affairs. — schopenhauer1

How can any state of affairs be better if no one can experience them?

How can any state of affairs be better if no one can experience them? — Pinprick

I think that is something he mentions as being objectively good. See here:
http://belmont.bme.umich.edu/wp-content/uploads/sites/377/2019/06/3.-Every-Conceivable-Harm-A-Further-Defence-of-Anti-Natalism.pdf
i) The asymmetry of procreational duties:
While we have a duty to avoid bringing into existence people who would lead miserable lives, we have no duty to bring into existence those who would lead happy lives.
ii) The prospective beneficence asymmetry:
It is strange to cite as a reason for having a child that that child will thereby be benefited. It is not similarly strange to cite as a reason for not having a child that that child
will suffer.
iii) The retrospective beneficence asymmetry:
When one has brought a suffering child into existence, it makes sense to regret having
brought that child into existence – and to regret it for the sake of that child. By contrast, when one fails to bring a happy child into existence, one cannot regret that failure for the sake of the person.

Because we can meaningfully talk about what could happen. It is always good that pain does not exist (in lieu of the possibility that it could exist). It is not bad that pleasure does not exist (even in lieu of the possibility that it could exist). The asymmetry is in the intuition that non-existent pain is valuable whereas non-existent pleasure is valuable in relativity to someone.

Again, with your objection, you would want to create something in order for there to be values like pain such that not having pain can have value. That is intuitively absurd though. For example, I can punch person X in the face a few minutes from now. Ceteris Paribus (let's say the person didn't deserve this), it is always good that I am not punching person X in the face. Me punching that person does not exist yet as an actuality, but certainly the fact that the potentiality of it being prevented is a good thing anwyays.