Well the usual EHT supply for a xerox machine or TV is designed for a very low current drain.
About 1 milliamp at most in a large tube TV.
So the internal resistance isn't a big deal as the voltage drop will still be low enough.
Also the power level has to be as low as possible for efficiency. There is no point in making a
power supply of 1 Amp rating when all you need is a milliamp or 2.
Finally the reason is safety, if someone hooks themselves onto the EHT its voltage will
hopefully collapse and give then some chance of surviving the experience.
BTW very early TV set's EHT did have a lower internal resistance as it came directly off a
mains transformer/rectifier. The famous Australian Radio Ham Ross Hull was killed that way just
before WW2 when he put his hand in the wrong place!
Regards Cliff Wright ZL1BDA ex G3NIA.