It's even more interesting when you think that that video is ten years old and she still teaching at 95, looking pretty much the same...
That's my personal ambition - to keep working until I die.
The reason people do all kinds of harmful things is usually that at some point in the past, in some way or another, it "worked" for them, it worked well enough. People generally try to the best they can with what they have and what they know... A lot of habits are learned a s a child, imitating the adults around you. You can see little girls standing in a "feminine" posture imitating grown up women who wear high heels for example years before they will wear the heels themselves. Also, " good posture" generally seems to translate into lifting the chest up, and most of us want to have "good posture", right? It's a very insidious idea I find.
In the end the "why?" doesn't matter that much I find. Often once people have learned a new way of standing and moving, they even find it difficult to recall the old one...