Semiconductor manufacturing involves a lot of precisely controlled processes. You put the wafers into furnaces and pass a gas over them for X time and Y flow rate at Z temperature to impregnate the wafer with various chemicals. You put them in low pressure plasma environments to etch them, again for X time at Y flow rate. There are half a dozen more of these as well, like applying metal and implanting ions.
These values are experimentally tightened to get the highest possible accuracy to the desired effect and improve the number of working chips that leave the factory. If the power cuts out you don't know what conditions the wafer experienced while the system was winding down completely uncontrolled and your processes haven't been designed for the wafer going through the ramp up twice.
The reason why it's lost so much output is because modern semiconductor processes have hundreds of steps and (I believe) a lead time in the months, so the amount of material that's in flight at any one instant has to be huge to get any reasonable throughput.
These values are experimentally tightened to get the highest possible accuracy to the desired effect and improve the number of working chips that leave the factory. If the power cuts out you don't know what conditions the wafer experienced while the system was winding down completely uncontrolled and your processes haven't been designed for the wafer going through the ramp up twice.
The reason why it's lost so much output is because modern semiconductor processes have hundreds of steps and (I believe) a lead time in the months, so the amount of material that's in flight at any one instant has to be huge to get any reasonable throughput.