Traditional publishing provides lower royalties because publishers invest a great deal in the books they publish. All the things self-publishers have to find and pay for themselves traditional publishers provide and pay for: developmental AND copy editing, design, professional cover illustrations and design. They provide basic marketing with a potential for being reviewed in professional journals and sold in bookstores, though those things don't always happen. But they rarely happen for self-published books.

Traditional publishing is difficult for writers to get into and it isn't always successful. Many traditionally published books don't do well. But traditional publishing companies often don't get credit for the effort they put into the books that they do agree to publish.