Why are there such objects as fake books, that is, empty shells? Why not put a real book in this space?

Fake books, as the name implies, are books that don't need to be read. Mostly used in film and television photography. In some specific circumstances, books are needed as the background, such as the study in the studio. In the case that the purpose is not to study but to decorate, buying fake books will cost much less than buying real books. In contrast, fake books are often used when decoration is needed.

Generally speaking, if you still don't understand, you might as well ask.