What might be going on is that people are tempted to use a person's preference for existing as a proxy for the value of their life, in the same way that a person's preferences for birthday presents can inform us about what kinds of birthday presents will make them happier.

I would certainly think twice about having a child if I knew the child would grow up to express a wish to never have been born. And I'm not even a preference utilitarian. But this approach seems problematic, and it's probably better to just ask ourselves what kind of people we want to bring into existence.