even if you read only fiction
Isn't this post specifically about fiction? When we say that a person "reads", it normally means fiction. Plus, I don't think anything else is typically measured in number of books.
I guess what I mean to ask is: what we can gain from reading works of fiction over other forms of text? Would you give the same answer given the clarification?
What else do you read that can be measured in number of books? You wouldn't do that for news articles, blog posts, or scientific papers. Cookbooks, textbooks and dictionaries are books, but you rarely read those from cover to cover, so you wouldn't see people talking about the number of books they've read in that context.