How do you know what life was like 2000 years ago? I don't think you can truly know when this convention appeared. I suspect it's tied to urbanism at least. If you're living alone in the woods, miles from anywhere, and someone walks past your house, you're probably not going to politely ignore them.

I think creepiness manifests when the observation is one way. Without technology that’s kind of hard. With tech it becomes increasingly easy for everyday people to one-way spy on each other