J. S. Bach had a killer method book for organists started. Then, after a few tables, and some rules about figured bass, it broke off with the line "this can only be delivered orally." Bach apparently decided it was too complicated to try to get all of that information into a book, and it would be better to just teach an individual student face to face and with words from his own mouth.
Whether this was partially a cop-out, there is certainly wisdom in the face-to-face encounter. A lot of questions can be answered, there is no sense of one-size-fits-all; you can react to a students particular needs, strengths, deficiencies, and speed of progress. Many of us dispense knowledge on the internet, and while it is certainly tempting to want to read something for free and do it at one's own pace and in one's own way, without having the responsibility of responding to another human being, it can obviously lack something. And being sure you can really get a student from point A to point B when you barely have any contact with them is surely a hazardous way to go about it.
People ask me questions about piano playing all the time via email and the questions page and I usually feel that it is not optimal to have never met them or heard them play, although I may be able to make a pretty good guess at what they need to know and what their skill level is just from what they've told me (or how). But I usually wind up wishing (and probably recommending) that they have a teacher, a human being in the room with them (or at least on Skype) who can give feedback to what they present and find a way (and that often requires a virtuosity of teaching strategies) to unlock something in them that they didn't notice on their own.
Of course, being in the hinterlands with no good teachers and an internet connection invites the resources the web can provide. Even with hundreds of hours of lessons I have myself found useful videos that I can watch and learn from great musicians. It helps, though, if you know what you are looking for.
Back to what I said last week. The part of practice where you diagnose, and propose a solution, and go about improving each spot, in short, knowing how to effectively practice: that is why we need teachers. Having studied with so many good ones for so long I am generally able to be my own teacher these days. But even established musicians, before giving important concerts, like to have some personal feedback. It's not likely to be in the area of correcting notes and posture, but all the same it is the type of thing that only a trained human being will notice, and it will come through observation.
Observation, and adjustment. That never ends. Whether you are listening carefully to every nuance you play, or your teacher is. And until you are able to do it yourself, you need a teacher.
Think about it, young people. Of all ages.