What makes you say that piano can't be taught over Zoom? I know a few music teachers that teach instruments remotely - though not piano, I'm not sure why that would be different?
I don't play the piano myself, but my understanding is that effective teaching requires constant feedback (including, possibly, physical correction) on both your posture and your hand positions/motions in a way that really requires the teacher to be present in the same room. I've heard this both from piano players and teachers.
No idea. I do play piano, but it was such a long time ago that I got lessons, it's hard to remember what was taught. I think whatever I was told about posture definitely didn't sink in though.
