I agree that white space is important. But why have so much of it on the OUTER edges of the page, rather than in the inner margins. When you read a book the paper on the inner margins is typically curved, and thus the "effective white space" you see there is less than you could conclude from these flat illustrations.

I can see one reason which could have historic reasons. When you flip the pages with your hands, they will wear out more on the outer horizontal margins. If you make that margin wider there is less chance of corrupting the printed content. However that would not apply to online pages.