I would thing a large portion of it is actually sound. The microphone in a laptop is just not very good, and then even if the instructor has expensive studio monitors its going to get super compressed. On top of the bad acoustics in whatever random room they are in, and background noise.

And while a multi camera setup would certainly be awesome, and help with positioning of hands and instrument, its just not really a practical setup for the majority of people. Now you need a great microphone, multiple cameras, probably tripods or something for camera positioning. An in person class solves all those problems.

45 minutes seems to be the standard for most classes, and I believe the research is as classes go longer than that they become less effective for learning. And sure you can do group classes, that happens in band class, but the student does not get as much attention and depending on the learning style and student that may just not be enough.