In the context of SCADA systems, why is a master terminal unit configured to prioritize 'exception-based reporting' rather than constant polling of all digital status points?

A SCADA system uses a master terminal unit, which is the main computer that monitors and controls remote equipment, to keep track of things like switches or sensors. If the master unit uses constant polling, it must constantly ask every single sensor if anything has changed, even if everything is perfectly normal. This is like a teacher constantly walki....