Well, part of it is that the amperage isn't something you want to bother calculating. If you're pumping 4000 volts to a streetlight... actually, a lot of street lights, you have to be able to handle the load of a bunch of florescant lights powering up at once. The amount of amperage that the line can provide varies, based on if the other lights are on, which wires you're touching, how much of your body you've got as the Load, etc. The line will always have 4000 volts (roughly), but the current can't be nailed down as easily.As for taking 50,000 volts... if the power supply can only handle 1 Watt, then 50,000 volts isn't that big a deal. For the most part, with electricity, you put a voltage on a line, have components that can produce a given wattage, and let the load draw whatever current it needs based on its resistance.
