I always come back to this:
The traditional methods of teaching musical instruments have been developed over the last 200+ years, and they are used worldwide to get hundreds of thousands of bored inattentive middle schoolers playing at a functional level quickly and with the minimum of bad habits. While, of course, it's true there are many people who learn to play at a functional (or even a high) level by going some other route, why wouldn't you just go right down the good ol' garden path that's been well-trodden for all those years? The chances that you're going to develop bad habits or be unable to get certain things at all, is pretty high in self-teaching things like how to play a musical instrument; why plan a course where failure is a high probability?
I will say that I can tell upright bass players who've "taught themselves" rather quickly; puny little tone caused by picking at the strings up around chest level; no endurance when playing in flat keys; weird left hand shapes that poop out quickly and require twice as many shifts as necessary; dozens of dots and stickers all up and down the neck. That doesn't take into account all the people I talk to who OWN a bass, but never play it because "it's too hard" - well, no, it's not really "Too hard" if you learn how to address the thing properly and use proper technique. I can play a three hour gig in flat keys and come away with only normal fatigue, and I don't practice dozens of hours a week, either.
Few people, wanting to learn how to play golf, would decide a reasonable course would be to teach oneself by going to the batting cage and swinging at baseballs. Most people who want to learn how to play golf go down to the driving range or the public course and engage the local golf pro for some lessons.