After a decade of teaching karate and practicing for 40 years, i have learned that it is very important for the teacher to be able to see every little wrong angle in the body of a student when performing anything from a strike to a simple stance. Medically speaking, a wrong position can result in injury or at the very least be a bad habit that can be a deterrent in the proper progress of things later on.
Unless we have a high resolution camera and a very large monitor, it will be difficult for a teacher to see these small things. I also have to move a lot to see things at a certain angle. This is the reason why i decided not to teach classes online. Correcting these bad habits at a later date is very difficult. It would be better to just spend the time correcting them in the beginning.
Why are there seminars being held online? This is often due to the fact that the instructor is foreign and possible because the attendees are all high in rank that the basics are not monitored anymore.