Yeah, NZ & Aus both have a ‘standard drinks’ system.

My guess is that larger quantities of alcohol (particularly bottles of spirits but also wine) simply aren’t intended to be drunk by one person in one sitting. Total volume of alcohol isn’t that useful; it’s more useful to be able to work out how much is in one shot or one glass.

This is especially important when you look at the same product being sold by the shot/bottle/cask/barrel, or being able to buy a gallon of it in your own container historically.