Quoting Scarlett.Harlot.Starlet.:" Exactly and crazy people don't seem to know they're crazy so they aren't going to seek help themselves."
A lot of people will also ignore the signs and not try to help someone who needs it. There are many who slip through the cracks. Also there is a stigma that makes people who need help afraid to ask. For example, I never told people directly in high school I wanted to kill myself, but I did invite my parents and best friend to read my journal that did say that. They all said no, I'm sure there were other signs too.