Why doesn't citation of a book give specific pages? 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.
It's always good practice to cite as closely as possible, especially in a book. LaTeX makes it easy to do this with the \cite[]{} form. For a paper, it's often not necessary since one typically cites the main result of a paper. But even with a paper, if what you're citing is a lemma buried inside (and that is not obvious from the abstract) it's good to say [23, Lemma 3.1] or something like that. 

As to why people don't do it, AnonymousMathematician already answered that above.