As an example I assume you're familiar with the treatment family of Sandy Hook victims have gotten? Where they have to deal with constant harassment and threats from people that believe it is a hoax and those that encourage them?

If you were to slap a content disclaimer saying 'this content is fake', they would just use that as fuel and proof that the government doesn't want you to know the truth. The lie directly harms people by simply existing even if you add a disclaimer.