Why can't electronic systems use constant current power source and vary the voltage as power consumption varies?
While there's no reason why you couldn't design electronics with constant current sources, there's a few good reasons why we don't. Batteries and mains power are supplied more as constant voltage sources than constant current sources so it's simply more convenient to use what we have. The other reason is that a constant current source is always burning power. To shut a device off, you would have to short-circuit the power-supply. Since most wires have resistance, you'd constantly be wasting power.