You realize most of those stories were originally books? They turn books to mangas and then to animes.
Apothecary Diaries started as a light novel, but JJK, AoT and many others start as Manga
> No, not all start as light novels
Many are though, many of the animes that came out for a few years I had already read the LN for. That so many novels becomes animes is likely a big reason why there are so many novels being written along those styles.