2000000 of anything sounds like a lot, so why not use prefixes to simplify?
That’s why you either use kcal (or Cal) or kJ, but not Mcal (or kCal, which is easily confused with kcal) or MJ, because most things you eat and drink are between 0 and a few hundred kcal. This way you have one unit and keep it consistent instead of switching between kcal and Mcal all the time or saying awkward stuff like you ate something that only had 0.004 Mcal.
Europeans use kilojoules to the same effect.
While kJ is required in labeling in Europe most people still use kcal for everything. AFAIK the only country somewhat consistently using kJ is Australia (the one with the kangaroos).
That’s why you either use kcal (or Cal) or kJ, but not Mcal (or kCal, which is easily confused with kcal) or MJ, because most things you eat and drink are between 0 and a few hundred kcal. This way you have one unit and keep it consistent instead of switching between kcal and Mcal all the time or saying awkward stuff like you ate something that only had 0.004 Mcal.
While kJ is required in labeling in Europe most people still use kcal for everything. AFAIK the only country somewhat consistently using kJ is Australia (the one with the kangaroos).