Transmission lines are not insulated at all. But that is not the problem. The problem is caused my the magnetic fields surrounding the conductors which is huge on the high voltage lines. If you remember from science class, to generate electricity you need a moving conductor and a magnetic field or a moving magnetic field and a conductor.
Because the power lines are AC the magnetic field is building, collapsing and changing direction with every cycle of the current.
Huge, strong, moving magnetic field + stationary metal objects and wiring in the house = generated electricity
The principal means of coupling is through the electric field. Figure a peak potential of maybe 300 KV, and a distance to ground of 30 Meters, and you have around 10 KV/M field strength right there.
The coupling due to magnetic fields is comparatively minor, and would fluctuate as the current being carried by the powerline fluctuated.
As far as biological effects, effects from alternating magnetic fields are more difficult to demonstrate than those from electric fields.
(Note that I am not talking here about electromagnetic radiation; only the separate effects of electric or magnetic fields need be considered in this case.)
I once lived under some 100+KV lines--about 70 feet distant--and the main effect was to make hum in my condenser microphones.