Because Yahoo! articles are THE SOURCE for all useful information:

Making a So-Called Weakness Seem Positive

Interviewers frequently ask candidates, "What are your weaknesses?" Conventional interview wisdom dictates that you highlight a weakness like "I'm a perfectionist," and turn it into a positive. Interviewers are not impressed, because they've probably heard the same answer a hundred times. If you are asked this question, highlight a skill that you wish to improve upon and describe what you are doing to enhance your skill in this area. Interviewers don't care what your weaknesses are. They want to see how you handle the question and what your answer indicates about you.