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).
Why do cycles damage supercapacitors? My understanding is that in batteries this is caused by ions not returning to their original spot in the electrodes, but I thought only the electrons moved in capacitors?
