Most books don't have complex layout. What they have is extremely high quality typography and careful attention to how the text appears on a specific page. Book designers tweak prose to avoid rivers (a series of whitespace on successive lines that happen to line up with each other), widows (the final line of a paragraph appearing on its own at the beginning of a page), orphans (the first line of a paragraph appearing on its own at the beginning of a page, or when the last line of a paragraph contains only a single word), and hyphenation.

The web, which doesn't have well-defined pages or even page sizes, precise control over typography, or designers who agonize over how the actual text appears in the context of the layout, lacks all of those. Typography is slowly getting better, but text layout algorithms are still relatively primitive.