A percentage is going to be more useful to a consumer than a volume anyway, especially for bottles that are consumed in part. 40% tells you a drink is going to be strong, without any further math based on whatever size drink you have poured yourself. If you wanted to know the volume alcohol of a poured drink, you’d have to linearly interpolate according to the size of glass you poured yourself, which is one step beyond just multiplying the alcohol percentage by the volume of the glass.

So in conclusion, labeling the volume of alcohol on a bottle is only practical for beer bottles and prepackaged cocktails, etc. If you do want to know the volume of alcohol consumed, simply multiply the volume of drink consumed by the ABV%.