>>12
Conservation of charge. Related to that is Kirchoff's Current Law and Kirchoff's Voltage law.
Basically you can't destroy or create electrons any more than you can destroy or create any other type of matter (under normal operating conditions). And its really hard to store an electric charge under the quantities and conditions you want to make it useful, and much easier to change work into electrical power. Think of a basic circuit with a voltage supply and a resistor. Then think of an analog equivalent: a water pump connected to a pipe with a waterwheel in the middle thats connected to the other end of the pipe. If you open up the "pipe" then the pump runs out of water really fast. But while the pipe might last a couple seconds, circuits scale down much more (atoms are smaaaalllll) and it would be milliseconds, if that.