They get that way. Similarly, communities around books that have been adapted can be pretty shitty too. r/brakebills became a total nightmare when The Magicians TV series became big, accepting the series blindly and almost wholly rejecting the trilogy of books it draws from. Those people claim to be fans, but some can be really shitty about the books. I was an OG fan from when the first book came out, was a member of the tiny little Facebook fan group from when it started - but when the series came out and the fangirls started squeeing over it, I was removed for "toxic behaviour" after saying I felt the adaptation missed the point the books were getting at. An admin even messaged me after removing me, claiming I was upsetting other fans.