Why do we use so many units for energy? (imgur.com)

You forgot to add the heat units, like BTU and calories, and the units for food like "kalories", and the units for air conditioners, like "frigorie".

Short short answer: because those voltages are "nominal", not what's really observed in-circuit. The available output charge (ampere*time -> charge) is much more controllable during manufacture, as it's directly related to the quantity of available reactants within the battery.