Why doesn't citation of a book give specific pages?
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.