I think people find the repugnant conclusion repugnant because they are using two different definitions for a "life barely worth living".

Society has very strong social norms against telling people to commit suicide. When someone's life is really miserable, almost no one tells them that killing themselves is the best thing they can do. Even euthanasia for people who are permanently and unavoidably suffering is controversial. So from a utilitarian perspective, you could say that people tend to have a strong "pro-life" bias, even when more life means more suffering.

But let's consider which lives actually have marginal benefit. Is it really actually a morally positive thing to bring in to the world someone who's life is going to be miserable?

Consider someone who is going to have an unpleasant childhood, an unpleasant adulthood, work too many hours at a job they don't enjoy as an adult, have their spouse die early, and finally die a lonely and isolated death themselves. Would you really bring someone like that in to the world given the choice? (Assuming no positive externalities from them working at their job.)

But let's say you meet this person in college and you can tell how the rest of their life is going to go. Would you encourage them to commit suicide, thereby reaping a moral surplus? Probably not, because now you're operating under the original definition of "life worth living" since the question is whether to kill instead of whether to bring in to existence.

Where is the "utilitarian zero point" actually at? In my view, the zero point represents someone whose life has ups and downs, and all the ups exactly cancel out all the downs. So now say we had a world where everyone's life has ups and downs, and for every person, there are ever-so-slightly more ups than downs--and there are tons and tons of people. That sounds pretty desirable, doesn't it?