BuddyPress.org

“It seems silly that a “universal” field like a profile field wouldn’t update it in all circumstances across user profiles. Why is that?”

Think of an option field as a stencil, and think of the database as the surface you paint onto through the stencil. When you change the stencil, it doesn’t automatically go back and repaint all the previous surfaces. You would have to go back and re-paint them yourself, which would be like doing an update on the database. No way to avoid it.