Because sometimes people do actually feel that way. Sometimes your life feels like it's caving in on you. Sometimes people really do feel like they don't exist, like they just want to curl up into a ball, and go into that place between life and death. Saying "I don't want to exist" isn't saying "I want to go die". It's saying "I wish that, for the time being, I could go somewhere and not have to feel". I don't think there's anything wrong with that. And if you don't know how it feels to feel this way, then you have no place to judge anyone who does.