One reason it is hard to write a book that does everything best, as you can do in a lecture, is that your book is supposed to be your version. So you can't just steal the best version of every topic and out them all in your book, as that borders on plagiarism, since the book will have your name on it. So you either have to just say "I am stealing this from book XXX" or you have to rewrite it according to your own grasp of it. Often however another existing book will already have done a certain topic as well as possible. Then you can't improve on it and you can't copy it and you can't leave it out, so you are forced to do it in a inferior way.
Of course there is a lot of borrowing in books, but this problem does not arise in a lecture, where you just freely take everything from anywhere you want. You will notice in my free algebra notes I often say I got something from another book, so i try to take the path of rewriting in my own way each topic from the best presentation I can find. Of course my version will not be as good as the classic presentations already out there, but to tell the truth I wrote mine to learn it myself, not so much to teach it to others. Some people write books to make money. Others write them to get out an accessible presentation of an important topic that so far has no such available text.
Every book has a different goal, which the author usually states up front and which can help you decide if his book is for you. Hungerford's book was written, he says, in order to be readable and understandable to the average (math phd) graduate student, (in 1970). Notice he does not say the goal is to present the deepest version of every topic. In my book, one goal was never to leave the student hanging at any point by saying "a straightforward computation left to the reader (which I myself was unable to do) completes the argument." I also tried to teach the student to understand how to use the basic theorems, not just prove them, especially ones it had taken me a lot of trouble to understand.
Mike Artin says his algebra book was written to emphasize the most important topic, namely linear algebra in all its guises. So you see you are getting something different from each person. And of course Mike's mastery of his subject comes through in original proofs of many things, that are simpler than traditional ones. In Hungerford's book, in line with his goal, he gives a careful logical presentation of each proof where each step follows clearly from the previous one. I borrowed from both these books, but my goal was to present proofs that give some insight into why the proof is correct rather than present one that is possible to trace all the way through, but that might still be opaque as to how it was thought of. So I tried more to to give away the idea behind all my proofs.