Interesting question. I try to stay away from "people are stupid", that's not constructive. People can be inconsistent in their beliefs, true. Maybe because they don't think about it too much, maybe they don't want to think about it, blindly stick to something for too long and they don't want to get rid of the belief that was a big part of thief life for so long (any belief, religion, ideology), admitting mistakes is uncomfortable, don't have time to think about it, worry about other things.
I question your claim of "most". I don't know if that is true or false.
Anyway, I came up with two possible reasons that could justify it:
1. Just because you assign a negative value to life, it does not mean you need to commit suicide. Since you are already here, you might as well "recover your loses". If you assigned a number of happiness to your life, it starts with -1,000,000,000,000,000,000,000,000, no matter what you do, not being born would have been better. But you can try to have a life, eat ice cream, watch movies, play games, do whatever makes you happy. You will die with a negative overall score anyway, but the closer you get to 0, the better.
2. If you think life has negative value, your goal could be to convince as many people to it as possible. The longer you live, the more people you can convince. It's an interesting ideology in the sense that theoretically these people should just die out naturally, but somehow they keep coming. Even if all antinatalists "died out", somebody could still come up with the same realization at some point.
I'm not necessarily showing my point of view, just trying to otherstand this strange world. I think a lot of things, ideologies can be justified in many ways, even if we find them silly or disagree with them. Each of us assigns different values to different things and it can lead to conflicting outcomes.