In the UK helicopters are used as drones simply aren’t fast enough to image an entire transmission line in a reasonable amount of time. However unlike PG&E we

In the UK helicopters are used as drones simply aren’t fast enough to image an entire transmission line in a reasonable amount of time. However unlike PG&E we take high resolution photographs of every major component on a transmission tower, and manually inspect and classify every image take. Images of this type would have identified this fault, at least once during inspections taken during the last couple of decades. Source: Worked for a company that did these inspections.