they DO monitor transmission lines like that. But it is not a simple thing to "turn off" the power in a hurry. For one thing, it is a giant estimation theory problem to even know what is happening. The transmission lines, act like distributed "transmisssion lines"...with reflections, time delays, frequency dispersion....Maybe they could aurmoatucally detect a severed line and within a second, send a command somewhere to cut the power to that area of the power lines. Not trivial, probably years to design and implement.
I remember a buddy went to the National Speleological Society party down south once. They had a GIANT bonfire, like 50' tall. It was nearby to a power line, and the heat from the fire made the line droop. The power company detected the droop and sent out a crew to figure out what was going on. And this was in the 1980's. Imagine today the sensor technology.