A lot of them don't read
books at all. They look at forum posts, wikis and D&D and assume they understand
Tolkien based on a 7th grader's shoddy interpretation.
The Silmarillion is not really what your average fantasy reader today wants to get into. The Silmarillion is fantasy for people who like mythology, not for people whose entire impression of fantasy is from derivative Tolkienisms and Harry Potter (nothing against Harry, but it's not exactly the Kalevala).
Addendum: Warhammer 40K fandom suffers from this a lot, too. There are tons of game books and
novels, but 40% of the people arguing on forums are simply repeating memes based on books they've never read. They literally don't know what they're talking about. It would be like you and I arguing about the plot of a movie we've never seen. Deranged.