It is because the world is full of ignorant twats
Once upon a time we got emails from friends, relatives, and acquaintances warning us about some nonsense or other. It is why scopes was created so that we could point them to the fact that some ill little girl did not want postcards, that having a system32 directory did not mean you have a very bad virus, and that Dihydrogen Monoxide was parody:
All of that bullshit has now transferred from email to facebook, and our penchant for passing on the weird remains undiminished.
Note: The gubmint is not spraying poison trails from aircraft, there are no secret cities buried under the Mojave Dessert, and Boris is not going to be putting an extra £350 million a week into the NHS.