But I still have to do something to control my time. A door isn't enough, because someone will just knock or open it. You have to put in effort and plan ahead to control your workflow.
A lot of people seem to believe that work should be easy. It's actually difficult and complex to work really efficiently. It requires knowledge of how to work efficiently, and then it requires a lot of people to stick to a process, someone to measure it and keep people on track, and continuously changing it to improve. That's a lot more than just "don't interrupt me".