Here's why:
* Quotation marks are used for specific purposes: They are used to indicate a title within a title, like a song title within a book title. For example, "The Song of the Lark" (the song title) is the title of a book by Willa Cather.
* Most book titles are not titles within titles: They are standalone titles, so they don't need quotation marks. For example, "Pride and Prejudice" or "To Kill a Mockingbird" are book titles, not titles within titles.
* Some titles do use quotation marks for stylistic reasons: This is more common in poetry or experimental works.
It's important to note that some people might use quotation marks incorrectly, but generally, they are not necessary for most book titles.