I think there are a number of arguments that you could make regarding people choosing not to commit suicide despite life being net negative, like fear, not wanting to cause harm to loved ones, etc. But the mere fact that suicide rates are not much higher suggests that people are not exercising an exit option. If lives were consistently and significantly net negative, I'd expect much more suicide.

Most people judge their lives to be worth living. It seems alienating and paternalistic to disagree with them. The right theory shouldn't be alienating, or at least not so alienating to frequently disagree with people on something this important about their own welfare.

FWIW, this position is compatible with person-affecting views, including asymmetric ones, as well as antinatalism and some versions of negative utilitarianism. A life can be worth living or continuing, without it being the case that it would be worth starting. I find preference-affecting views (including asymmetric ones) to be the least alienating.