Ah, but you see, he WASN'T! He was a Professor of Languages. Fiction writing was... a hobby, I suppose. He created languages for fun, then wrote backstory to explain the languages! That's where The Book of Lost Tales came from, which eventually evolved into The Silmarillion. The Hobbit was a bedtime story he created for his children, but the "Elder Days" kept forcing their way into the story... and when fans demanded more Hobbit stories, well, the connection back to the First and Second Ages could no longer be resisted.