> Why is it so easy to cast doubt about them?

The entire vaccines cause autism thing started as a disinformation campaign against the MMR vaccine in order to sell an alternative (expensive) 3 part vaccine and to win a lawsuit. The media gave the original paper huge coverage without actually reading it and it took many people a lot of work debunking and discrediting the author before he even lost his medical license.

You never read "We got tricked into spreading BS" on the front page of a newspaper.