Mostly money. I think lessons are very good for basics. Someone learning should get a couple right off. But regular lessons are a luxury and depending on who the instructor is can be pretty expensive. I think if I'm doing something and find more than a couple ways to do it, I'd want lessons just to make sure I'm exploring the correct options. Or if I have something specific I'm trying to achieve, then I'd want a lesson or two to get started correctly. Even better, teach me the basics and goals, such that I can be self-guided for a much as possible.
But, when you see a McLesson on TV by some of the world's best, and you just see them provide the same old drills, or some gadgety way to attack or look at something, then my impression on value-add just takes a nose dive.
I've had two, relatively cheap lessons and the input and drills were great and he said "go work on these this season, you have enough to play with for a couple months" - I really appreciated that, he helped my game and didn't try to turn it into a regular paycheck. I did the drills, did a followup lesson and got huge gains.
Conversely, I've talked (no lesson) to a couple very high end (expensive) instructors to scope out their methods, and they wanted to immediately retune my swing dramatically to something 'trendy' that actually caused physical pain in my wrists. Same guys have not helped a golf buddy at all.
So much depends on the instructor - However, if there is video and the occasional Trackman involved, then I'm ok with it. I'd still say that if someone can coach the basics only, then anything after that one can develop for themselves - provided they understand how the body works, and the physics involved. That's not "self taught", it's "put me on the right path and just provide course corrections occasionally while I drive the boat"