I suspect a large part of this is the one-two punch of the intertwining of art with capitalism and the totalising, alienating force of capitalism in our lives.
For most people, meaningfully interacting with the people who make the bulk of their art is all but impossible, and for popular (in the sense of having a lot of fans) artists, meaningfully interacting with the bulk of people who appreciate their art is also impossible.
In order to facilitate the having of lots of fans, the relationship between artist and fan has to be reduced to a money relation, to payment for content.
The problem with this, of course, is that content is the artistic equivalent of empty calories. It'll fill time, sure, it'll engage the senses, but it's limited by its role as product and commodity — content is for everyone, so it must be for nobody.
Enter The Fandom.
With the artist unavailable for participation, the next best thing is The Fandom. Whether a subreddit or a fan club, The Fandom becomes a surrogate for the artist, a community of real people that can actually be interacted with to engage with art beyond a surface level, actual people with (alleged) knowledge of The Themes and The Meanings of the art.
This, naturally, has the problems we're familiar with. When art is reduced to content, part of the process is having the edges sanded off. Content in large part becomes inkblot tests, half-formed images upon which the fan imposes their own meaning, creating an obvious contradiction — The Fandom, as a surrogate for the artist, requires orthodoxy; content, its messages weakened by endless committees and focus groups, breeds heterodoxy.
Acts of fandom, then, become purity tests and sigils of inclusion. Shaping participation in The Fandom into proscribed acts allows The Fandom to paper over the heterodoxy. After all, if two people have the same tattoo, for example, it ceases to matter if they believe the tattoo represents the same thing for both of them.
They both get to pretend it does, they get to believe they are seen and to believe they see in return, they get to feel kinship.
They get The Fandom.