Cause they're different, they're seen as weird freaks to "normal" people, I'm an otaku myself and I can tell you one thing it's all about stereotyping, people by nature belittle and insult anyone who is socially different then they are. We're seen as anti-social when in reality we just don't want to talk to them as we know they wont have any interest in what we have to say. In other words people fear what they don't understand and tend to shun it.
