Define “properly isolated”. From the electric current perspective, the cables are properly insulated, and the insulating material is air. But here what's at stake is electromagnetic (EM) waves generated by the oscillating current flowing throw the cables, and while EM shields can reduce these I'm not sure it would even be possible to shield an entire cable in order to contain the field, and even if it was theoretically possible, it's almost certain that it wouldn't be realistic at the scale of a power grid.