Why do people pick terrible 'Best Answers'? Examples included...?

It has happened to me a lot. I go through lots of questions and very often I see questions about books that not everyone reads and find that the only answer is, "I don't know" or "I haven't read the book" or "Do your own homework" and that's really irritating. I also get annoyed of the grammatically inaccurate answers, I mean why did they put spell check for? I am really glad that someone raised this question. Also about the voting, that's really stupid if you ask me, if you spend five minutes to ask a question, spend another five to pick a best answer! I also check with Wikipedia for some answers to be more accurate.