It is a "five"-point book.
Here are the books. They are all in piles by their scores. See, this one is a five. As in ' this one is a scale-five book.
Those over there are sevens.
Basically, any question about "Why" in grammar will get an answer that tries to help people learn it. Actually, it's just because that is how it is said. Grammar describes what actually happens, what is normally said.
Short answer: That is how to say it.
Does this answer get a seven out of ten?