Everyone I know is autistic then, which is plain impossible, so they're wrong at best. But, completely personal point of view:

Thinking that they do really cause autism is fun. It makes our lives more interesting. It shows that we are aware of dangers that most people aren't. It provides the means for us to exercise our critical thinking (doesn't matter if it's for something useful/true or not). It can emotionally reward our sense of protection.

As you see, there are many psychological factors that stimulate that belief. That's the reason why I don't outrightly dislike people who are against vaccines. It's like religion.