Verified answer

I think because as writers we want our characters to seem as appealing as possible. I guess we often forget that people can love a flaw almost as easy as they can a strength or good quality. We want our characters to be strong, usually. So we probably forget to give them flaws even though that could make the story even better.

Actually, humans are somewhat proud. Even the most humble get prideful. We like to give strengths because we probably want those strengths. Like a timid person writing about a brave person. I'm not saying this is how it is, but my opinion. You should read Lord of the Flies by William Golding; it gives great details on character strengths