The problem is that sitting in front of a computer gives no indication of business. You wouldn't interrupt a surgeon during an operation, or a pilot landing a p

The problem is that sitting in front of a computer gives no indication of business. You wouldn't interrupt a surgeon during an operation, or a pilot landing a plane, or a crane operator moving some load on a construction site, or a carpenter currently doing woodwork; because you can very visibly see them being focused on their work. Sitting in front of a computer is the default position of the modern office workplace, which makes it a meaningless signal.