They don't store any meaningful amount of electricity (which is why they haven't supplanted Li-Ion batteries for power tools) and more importantly they have a cycle-life - they degrade from usage (source: my wife works for a supercapacitor manufacturer).

You also get losses from practical usage - i.e. no one can build a 3V supercapacitor that has decent endurance (you can totally build one which will work, but you're rating it knowing that every cycle is damaging it).