[toomuchtodo]

Standing uses body language to convey authority and confidence when presenting. I’d theorize this helps the consumer of educational content in being engaged, which is more difficult when watching videos versus being present during an in person lecture.

[ldenoue]

but if the teacher is fully shown, or perhaps the upper body (in Japanese culture, people like to see the upper body during webcam sessions, not just the head), then it would still convey authority and confidence while watching the video?

Authority seems to come in part from the actual height difference, so that could be lost if the student watches a video... But standing still gives other benefits, see https://www.genardmethod.com/blog/bid/161977/Sitting-or-Stan...