Labeling alcohol content by %ABV or %ABW is more versatile than absolute measure. Its also trivial to figure the absolute content given a specific volume.

In the US at least, there are regulations for packaging and labeling enforced by a federal agency. (I think that agency is Alcohol, Tobacco and Firearms (ATF), but I could be wrong.) The label must include the %ABV or proof, and must also specify the volume. Every distiller and brewer must file a label for approval of every product and pay a free before the product can be packaged and sold. The fees are not trivial, and the approval takes time.

In the US, you can purchase liquor in bottles from 50mL all the way up to 1.75L, beer from 8oz (customary) to 750mL, and wine from 333mL up to a magnum (which volume I don't know off the top of my head), all with many sizes in between

Allowing for labeling the alcohol content in %ABV means the producer can package the same product in different sized units using the same approved label across the line. It saves on fees and administrative overhead.

edit to add: Another nuance of alcohol production and distribution in the US is bonding. There's a federal tax on all alcohol products made for consumption that is paid by the producer when the product leaves the production facility. Product is psckaged and held "in bond," a specific area of the facility where the product awaits sale and shipment. When it leaves Tue area, it must he recorded and taxes paid. It's serious business, and the ATF can sometimes pull licenses even for minor infractions.

To keep the math and record keeping easy, the producer tracks the number of units releaeed from bond, and usually reports and pays the duties on it quarterly. Since the duties are assessed by absolute volume, the duties are calculated by %ABV times times number of units.