For some people, Kate and William could knock on their door, be invited in for a cup of tea and a chat and it still wouldn't be good enough.
Feelings on the benefits or otherwise of the Royals are irrelevant to this storm of conspiracy - people are serious in their belief that this famous woman, who most people in the UK would recognise, has been done away with or is being held against her will, and "they" are putting out photos of lookalikes or photoshop or whatever.
It's like "Paul McCartney is dead" all over again. Just with more technology.