Text in a book is horizontal, also an opened book is most often wider than tall - so it's in landscape too. It's only in portrait when it's closed so it'll take up less space on the shelf - which is also horizontal.
You can use smartphone in landscape too, but it's not as convenient to hold, and read text. But for watching video, you bet I'll rotate it to landscape.
Not like English doesn't have words that mean funny in other languages ๐คทโโ๏ธ