In papers or books, citation to another book usually doesn't contain the specific page, section, chapter of where a result is borrowed. If the book is really thick, and the readers may have different knowledge levels and familiarity with the book, some readers may find it not easy to locate the borrowed result within the reference book. So why don't people specify the source of a citation as detailed as possible in books?
BTW, it is good to specify as detail as possible for citation to a paper. But since a paper is usually much shorter than a book and it is usually in a searchable electronic form, it may be much easier to find the source in a paper than in a book.