Why do insulated screwdrivers have a maximum voltage that they're rated for?
Wrong.
At some point, any insulation will break down and either begin to conduct or simply flash over.
From Wikipedia:  


  Electrical breakdown or dielectric breakdown is when current flows
  through an electrical insulator when the voltage applied across it
  exceeds the breakdown voltage. This results in the insulator becoming
  electrically conductive. Electrical breakdown may be a momentary event
  (as in an electrostatic discharge), or may lead to a continuous arc if
  protective devices fail to interrupt the current in a power circuit.
  
  Under sufficient electrical stress, electrical breakdown can occur
  within solids, liquids, gases or vacuum. However, the specific
  breakdown mechanisms are different for each kind of dielectric medium.


So, respect the rating on your 'insulated' screwdrivers.