Books never say 'sorry, not responding' or 'oops, an error has been found'.
Books are not fast sometimes and slow other times - they are perfectly matched to your own speed.
Books don't change when you're not looking.
Books don't flash advertisements or distressing charity appeals in your face when you are trying to concentrate on something else.
Books don't require an electricity supply. In fact books are what you turn to during a power cut.
Books don't get infected with viruses (silverfish are not the same).
Books stay the same. Of course that can be a drawback - websites are updated continuously - but it's comforting to pick up a book and know that the illustration I want to look at will be on the same page and in the same position as every other time I looked at it.
Books don't 'crash'. You can't lose all your books in an instant.
And ultimately, you can't curl up in bed with a good computer......