You are spot on in your assessment here about how people react. Problem is that it causes many drivers to feel self conscious and the horn doesn't get used enough.
No one likes to be confronted over this so they avoid the problem entirely.
You're spot on in this statement. I beep a few times when backing generally and that is it, not every 3 seconds unless I am backing close to a door where I know people frequently enter and exit or I am doing a particularly long back.
It sounds crazy but I become self conscious about doing it continuously. I bet this is the fact for many drivers. Case in point at my building's Airwall most drivers beep twice and that's it. Probably only new driver's beep continuously. The same goes with backing inside the building, most drivers beep twice and that's all.
I drove a rental box truck last peak and was so happy for the backup alarm. It was one less thing I had to focus on doing while backing.