Why is it that the high voltage amplifiers do not use shielded output cables?

How to Use Power Supply FAQ

Of course, using the shielded output cables for DC power supply has efficiencies in noise shielding and high-frequency component reduction based on the cable capacity. In the meantime, however, concerning AC power supplies, according to the output capacity with the shielding, the power loss or heat generation is occurred when the current flows in the cable. What is worse, following the capacity, the difference in phase could occur, and the upper bandwidth frequency could be deteriorated.
Taking into account of these aspects, the non-shielded cables are adopted for output as of the high voltage amplifiers .