I think development is a profession where it is not obvious when you are interruptible. In fact one of the best manager I had outright ask me "are you interruptible?", because she doesn't know if I am deep into a complex problem or just cleaning up stuff or waiting for my code to compile.

Manual labor also require focus, but when you are occupied tends to be obvious. If you are under a car, messing around with a tool in your hand, everyone knows it is not the best time to interrupt you. In safety-critical jobs, like in the case of pilots, there are mandatory protocols that explicitly forbit interruptions during critical moments.

People usually know enough not to barge in when the manager is having an talk with his higher up or when the salesman is negotiating with a customer unless it really is urgent.

But programmers just stare at the screen all day, and most of the work happens in their head, so how do you know if they can be interrupted or not. Bad managers assume "always", because themselves only stare at the screen during their "off" time, so they assume programmers are always "off".