A lack of insight, denial of the very fact that they are ill is a prominent feature in a number of mental disorders, most notably schizophrenia. I think that what you’re saying about people getting together online to share delusions is a factor, but I don’t think it’s because they used to have a sense that they were deluded. Rather, they were isolated. Someone who thought that power lines were controlling their thoughts had very little ability to spread that belief. They could shout on a street corner, print and pass out pamphlets, and that was it. Unless you were nuts and rich, you couldn’t really spread the “word” far.

Now that has changed, and everyone, however disorganized and deprived of resources, can publish their every thought. People who think they’ve been “gang stalked” and people who think that the government is behind contrails all have a megaphone to the world. As a “bonus” they also are able to just type their delusions into Google and find sites to confirm and expand them. The same network effect that connects hobbyists and enthusiasts of rare things around the world does the same for people with delusional frameworks.

It also does the same for Holocaust deniers, and general shitheads.