In the low frequency (audio frequency as opposed to radio frequency) and near field domain, electric and magnetic fields exist and act independently. Meaning that there can be electric and/or magnetic interference fields that need to be treated individually.Why doesn't a humbucker's shielding cover stop the pickup from working? if it's blocking electromagnetic fields from causing interference with the pickup, why isn't it stopping the magnetic fields from the moving string being able to be picked up by the coils?
Thin electrical conductors like copper, brass, aluminum, nickel-silver and even conductive paint effectively shield electric interference fields (when grounded) but are ineffective regarding low frequency magnetic fields. That's why the humbucker was invented.